Note, not all `alexellis/github` references should be changed, there are
a number of repos which are not part of the openfaas org, this commit
excludes those.

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

This adds support for private docker registries, by adding
an optional `registryAuth` field in the CreateFunctionRequest.
Auth must be passed as base64-encoded basic auth, similar to
how done in Docker file store credentials (~/.docker/config.json).
Credentials are then passed to swarm at service creation.