Add support for user groups

This documents the v3 api changes proposed to represent support
for groups of users in keystone.  These changes match the
origional spec that was part of the blueprint.

blueprints.launchpad.net/keystone/+spec/user-groups

DocImpact

Change-Id: I48d874ce5b5019e5ddee56c9b9b73a796b2cbdff