Add detailed federation configuration docs

Add documentation on how to configure federation in keystone

Co-Authored-By: Steve Martinelli <stevemar@ca.ibm.com>

DocImpact
Closes-Bug: #1311693
Change-Id: Ia7e5a67d762921a81ef4a7d16c682e5db1c3ba6c
Merge "Add detailed federation configuration docs"