Support authentication via SAML 2.0 assertions

This patch will support authentication via SAML 2.0 assertions.
A new authentication plugin will allow external users to authenticate
with keystone, provided the incoming assertion is valid.

The file keystone/contrib/federation/controllers.py was extended with two
new controllers.V3Controller classes:

*) DomainV3 which handles /v3/OS-FEDERATION/domains API call and returns
   list of domains a user can access based on the provided list of groups.

*) ProjectV3 which handles /v3/OS-FEDERATION/projects API call and returns
   list of project a user can access based on the provided list of groups.

Change-Id: I89f70e3a24e825e21580772c088c6fd5c44f3b63
Implements: blueprint saml-id
Merge "Support authentication via SAML 2.0 assertions"