Federating multiple Keystones

This spec provides support for federating multiple Keystones.

bp keystone-to-keystone-federation

Co-Authored-By: Marek Denis <marek.denis@cern.ch>
Co-Authored-By: Steve Martinelli <stevemar@ca.ibm.com>
Co-Authored-By: David Chadwick <d.w.chadwick@kent.ac.uk>
Co-Authored-By: Kristy Siu <kwss@kent.ac.uk>

Change-Id: I648e6a9a18936c28c5104ae56c1127ac980be5e3
Merge "Federating multiple Keystones"