Routes for Keystone-IdP metadata endpoint

Keystone acting as an Identity Provider should expose an endpoint
where federated peers can access its SAML2 Metadata file.

API spec: https://review.openstack.org/#/c/118074/

Change-Id: I94d26ce88d2468af71bc1c9d1cba1b422da4480e
Implements: bp keystone-to-keystone-federation
Merge "Routes for Keystone-IdP metadata endpoint"