[Python] Add authentication links to Getting Started instructions

[bjwatson]

What:

Add more authentication links to http://pythonhosted.org/gax-google-pubsub-v1/starting.html#authentication:

• Application Default Credentials
• Also gcloud-python has excellent auth documentation

  Why:

So that users can understand how authentication works.

How:

This is controlled by templates/gax/python/docs/starting.rst.mustache.

[geigerj]

Note: in link http://pythonhosted.org/gax-google-pubsub-v1/starting.html#authentication, the third code block (to gcloud beta auth application-default login) is broken