Refactor: move federation functions to federation utils

Some of the functions created in keystone.auth.plugins.saml2 can be
re-used for other federation protocols that are supported in the future.
Propose moving them to keystone.contrib.federation.utils.

Change-Id: Ifb7adcda44a0eff2650198353393aff33b3eda7a
Merge "Refactor: move federation functions to federation utils"