Add initial Python SDK docs to user guide

Initial addition of Python SDK content to user's guide. Describes
how an end-user can employ the Python bindings to automate tasks.

This initial commit adds info on:
 - How to authenticate with Identity, Commpute, Image, and Network
   clients
 - How to manage images

Change-Id: Ie8c4120acc7739c0bc4bddd99ffdbfbfbe241e0f
Merge "Add initial Python SDK docs to user guide"