IETF ABFAB federation protocol.

bp abfab <https://blueprints.launchpad.net/keystone/+spec/abfab>

Co-Authored-By: David Chadwick <d.w.chadwick@kent.ac.uk>

Change-Id: Ie28aef4ad10c4eb1b5aecccd72a2fe8d7c78db3c
Merge "IETF ABFAB federation protocol."