Add guide for OpenFaaS on DigitalOcean (Swarm)

This commit adds a guide to getting started with OpenFaaS running on
DigitalOcean, it includes simple single node swarms up to multi-node
behind a DigitalOcean load balancer.

Signed-off-by: John McCabe <john@johnmccabe.net>

guide/deployment_digitalocean.md

+# Deployment guide for DigitalOcean
+
+## Create DigitalOcean API Access Token
+Follow DigitalOceans instructions to create a Personal Access Token with **Read** and **Write** permissions, give the token a descriptive name for example `openfaas-getting-started`.
+- https://www.digitalocean.com/community/tutorials/how-to-use-the-digitalocean-api-v2#how-to-generate-a-personal-access-token
+
+Set an environment variable with your token value.
+```
+export DOTOKEN=738cb0cd2jfhu84c33hu...
+```
+
+## Install Docker Machine
+Follow Dockers instructions to install Docker Machine on your laptop/desktop.
+- https://docs.docker.com/machine/install-machine/
+
+## Create Docker Nodes
+Use Docker Machine to create Docker nodes, the example below creates 3 droplets in the NYC3 zone, if you want to deploy only one Droplet change `"1 2 3"` to `"1"`.
+
+This process will take a few minutes as Droplets are created and Docker installed.
+```
+for i in 1 2 3; do
+    docker-machine create \
+        --driver digitalocean \
+        --digitalocean-image ubuntu-17-04-x64 \
+        --digitalocean-tags openfaas-getting-started \
+        --digitalocean-region=nyc3 \
+        --digitalocean-access-token $DOTOKEN \
+        node-$i;
+done
+```
+List the newly created Docker nodes.
+```
+docker-machine ls
+```
+```
+NAME     ACTIVE   DRIVER         STATE     URL                          SWARM   DOCKER        ERRORS
+node-1   -        digitalocean   Running   tcp://104.131.69.233:2376            v17.07.0-ce
+node-2   -        digitalocean   Running   tcp://104.131.115.146:2376           v17.07.0-ce
+node-3   -        digitalocean   Running   tcp://159.203.168.121:2376           v17.07.0-ce
+```
+
+Refer to the [documentation](https://docs.docker.com/machine/drivers/digital-ocean/) for more detailed information on the DigitalOcean options for docker-machine.
+
+# Initialize Docker Swarm
+Intialize Docker Swarm on `node-1`.
+```
+docker-machine ssh node-1 -- docker swarm init --advertise-addr $(docker-machine ip node-1)
+```
+If deploying only `1` node skip to the next section.
+
+If deploying `>1` node take a note of the command to add a worker to the Swarm. 
+```
+Swarm initialized: current node (je5vne1f974fea60ca75q2cac) is now a manager.
+
+To add a worker to this swarm, run the following command:
+
+    docker swarm join --token SWMTKN-1-239v0epdnhuol2ldguttncoaleovy29hnwyglde0kba1owc9ng-9488z5we2invwcn69f5flq7uu 104.131.69.233:2377
+
+To add a manager to this swarm, run 'docker swarm join-token manager' and follow the instructions.
+```
+
+Add `node-2` to the Swarm, using the `docker swarm join..` command returned when initializing the master.
+```
+docker-machine ssh node-2 -- docker swarm join --token SWMTKN-1-239v0epdnhuol2ldguttncoaleovy29hnwyglde0kba1owc9ng-9488z5we2invwcn69f5flq7uu 104.131.69.233:2377
+```
+Repeat for `node-3`.
+```
+docker-machine ssh node-3 -- docker swarm join --token SWMTKN-1-239v0epdnhuol2ldguttncoaleovy29hnwyglde0kba1owc9ng-9488z5we2invwcn69f5flq7uu 104.131.69.233:2377
+```
+
+## Configure Docker CLI to use remote Swarm
+Run this command each time you open a new shell, this tells Docker where your remote Swarm is.
+```
+eval $(docker-machine env node-1)
+```
+
+## Deploy the OpenFaaS Stack
+```
+$ git clone https://github.com/alexellis/faas && \
+  cd faas && \
+  git checkout 0.6.5 && \
+  ./deploy_stack.sh
+```
+
+## Test the UI
+Within a few seconds (or minutes if on a poor WiFi connection) the API gateway and sample functions will be deployed to the Swarm cluster running on DigitalOcean.
+
+Access the Gateway UI via the IP address returned by `docker-machine ip node-1` (you can also access via `node-2` and `node-3`):
+```
+echo http://$(docker-machine ip node-1):8080
+```
+Prometheus metrics can be viewed at:
+```
+echo http://$(docker-machine ip node-1):9090
+```
+
+## Deleting OpenFaaS Droplets
+You can use `docker-machine` to delete any created Droplets if are finished with your OpenFaaS deployment.
+```
+docker-machine rm node-1 node-2 node-3
+```
+
+## Creating a Load Balancer
+Rather than address each node individually it can be preferrable to run behind a load balancer.
+
+From the DigitalOcean console Networking page, open the Load Balancers tab and click *Create Load Balancer*.
+
+Give the balancer a name and select the Droplets which will be balanced using the `openfaas-getting-started` tag and `NYC3` region (these were values passed to docker-machine when creating the nodes).
+
+![create_lb](https://user-images.githubusercontent.com/83862/30240233-274c4dc0-9564-11e7-8881-54bce652392f.jpg)
+
+Update the forwarding rules to point at the Gateway on `8080` and Prometheus dashboard on `9090`:
+![forwarding_rules](https://user-images.githubusercontent.com/83862/30240106-0eb71242-9562-11e7-846e-093627026a7c.jpg)
+
+Expand the Advanced section and update the health check to use port `8080`.
+![health_checks](https://user-images.githubusercontent.com/83862/30240104-0e98e3d0-9562-11e7-89b6-c266384e35d8.jpg)
+
+Click `Create Load Balancer` and after a few minutes your balancer will be available.
+
+![balancer_ready](https://user-images.githubusercontent.com/83862/30240232-2747becc-9564-11e7-867a-c3ac220f2ae3.png)
+
+You can now access the OpenFaaS Gateway, Prometheus dashboard and all functions via the load balanced IP address. For example from the balancer above:
+- Gateway: http://45.55.124.29:8080
+- Prometheus: http://45.55.124.29:9090
+
+## Costs
+
+The costs of running your own OpenFaaS platform on DigitalOcean depend on how resiliant you want it to be, from the simplest single node deployment, to a three node swarm behind a load balancer.
+
+| Deployment Type | Resources | Monthly Cost | Hourly Cost |
+| --------------- | --------- | ------------ | ----------- |
+| Single Node Swarm | 1x 512MB/1CPU Droplet | $5 | < 1 cent |
+| Three Node Swarm | 3x 512MB/1CPU Droplet | $15 | ~ 2 cent |
+| Three Node Swarm with Load Balancer | 3x 512MB/1CPU Droplet + Balancer | $35 | ~ 5 cent |
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