IdP SAML Metadata generator

Keystone acting as a SAML2 Identity Provider needs to be able to
generate and expose metadata that is consumed by federated
Service Providers.

Co-Authored-By: Steve Martinelli <stevemar@ca.ibm.com>
Change-Id: I9e4b2f068a8190215749b95f31d634eb09c1e3f1
Implements: bp keystone-to-keystone-federation
Merge "IdP SAML Metadata generator"