Add SAML generation route to OS-FEDERATION

Create the specification for SAML generation in the OS-FEDERATION
api spec.

Co-Authored-By: Marek Denis <marek.denis@cern.ch>

Implements: bp keystone-to-keystone-federation

Change-Id: Ic0e062fb593e8251efb956dd6510c6eca71e59b3
Merge "Add SAML generation route to OS-FEDERATION"