Add v3 openstackclient CLI examples

Add some notes about authenticating with v3 keystone and
openstackclient. Also add some examples that don't exist in v2.0,
like domains and groups.

Change-Id: I92f9f9ab3ed4657f0771ad284ee6c4c613eca27c
Merge "Add v3 openstackclient CLI examples"