Add assertion mapping extension (Federation pt2)

Add an extension to manage assertion mappings, this is
required for adding federation support to keystone.

Co-Authored-By: Kristy Siu <kwss@kent.ac.uk>
Co-Authored-By: David Chadwick <d.w.chadwick@kent.ac.uk>
Co-Authored-By: Joe Savak <jsavak@gmail.com>
Co-Authored-By: Marek Denis <marek.denis@cern.ch>
Co-Authored-By: Dolph Mathews <dolph.mathews@gmail.com>
Co-Authored-By: Morgan Fainberg <m@metacloud.com>
Co-Authored-By: David Stanek <dstanek@dstanek.com>
Co-Authored-By: Brad Topol <btopol@us.ibm.com>

Implements blueprint mapping

Change-Id: I22cf500772ea1abde21ca1caf7dc8b16bfb06625
Merge "Add assertion mapping extension (Federation pt2)"