[SPARK-13041][MESOS] Adds sandbox uri to spark dispatcher ui

## What changes were proposed in this pull request?
Adds a sandbox link per driver in the dispatcher ui with minimal changes after a bug was fixed here:
https://issues.apache.org/jira/browse/MESOS-4992
The sandbox uri has the following format:
http://<proxy_uri>/#/slaves/\<agent-id\>/ frameworks/ \<scheduler-id\>/executors/\<driver-id\>/browse

For dc/os the proxy uri is <dc/os uri>/mesos. For the dc/os deployment scenario and to make things easier I introduced a new config property named `spark.mesos.proxy.baseURL` which should be passed to the dispatcher when launched using --conf. If no such configuration is detected then no sandbox uri is depicted, and there is an empty column with a header (this can be changed so nothing is shown).

Within dc/os the base url must be a property for the dispatcher that we should add in the future here:
https://github.com/mesosphere/universe/blob/9e7c909c3b8680eeb0494f2a58d5746e3bab18c1/repo/packages/S/spark/26/config.json
It is not easy to detect in different environments what is that uri so user should pass it.

## How was this patch tested?
Tested with the mesos test suite here: https://github.com/typesafehub/mesos-spark-integration-tests.
Attached image shows the ui modification where the sandbox header is added.
![image](https://user-images.githubusercontent.com/7945591/27831630-2a3b447e-60d4-11e7-87bb-d057efd4efa7.png)

Tested the uri redirection the way it was suggested here:
https://issues.apache.org/jira/browse/MESOS-4992

Built mesos 1.4 from the master branch and started the mesos dispatcher with the command:

`./sbin/start-mesos-dispatcher.sh --conf spark.mesos.proxy.baseURL=http://localhost:5050 -m mesos://127.0.0.1:5050`

Run a spark example:

`./bin/spark-submit   --class org.apache.spark.examples.SparkPi   --master mesos://10.10.1.79:7078   --deploy-mode cluster   --executor-memory 2G   --total-executor-cores 2     http://<path>/spark-examples_2.11-2.1.1.jar  10`

Sandbox uri is shown at the bottom of the page:

![image](https://user-images.githubusercontent.com/7945591/28599237-89d0a8c8-71b1-11e7-8f94-41ad117ceead.png)

Redirection works as expected:
![image](https://user-images.githubusercontent.com/7945591/28599247-a5d65248-71b1-11e7-8b5e-a0ac2a79fa23.png)

Author: Stavros Kontopoulos <st.kontopoulos@gmail.com>

Closes #18528 from skonto/adds_the_sandbox_uri.