Enforce authenticated=False in saml2 plugin

All underlying HTTP calls executed via ``keystoneclient.session.Session``
object should have ``authenticated=False`` option enforced indicating the
plugin is not authenticated with the Identity Service yet.

Change-Id: I946f1ed6a55c4172d8f4bf6a24e5cbc3a00d1154
Merge "Enforce authenticated=False in saml2 plugin"