We do currently not support www_authentication_uri at all, which
is the new standard, as auth_uri has long been deprecated.
* Make sure we support both auth_uri and www_authenticate_uri.
* Switched to www_authenticate_uri for devstack.
* Fixed a bug where a bad exception would be thrown if auth_uri
was not set.
Story: 2004271
Task: 27819
Change-Id: Ibc932d35f3d6ba2ac7ffb6193aa37bd4a3d4422e