Adding authentication compatibility for OpenStackClient

In ksa, plugins loading is managed by ksa.
OpenStackClient token_endpoint plugin expects get_auth_ref[1],
(a method consistent with ksa auth plugins) which is not implemented
in ksa token_endpoint.

This change will not change overall ksa behavior, as it only makes
OpenStackClient to be able to provide its token_endpoint plugin,
necessary for authentication migration in osc from ksc to ksa[2].

[1] https://github.com/openstack/python-openstackclient/blob/
master/openstackclient/api/auth_plugin.py#L48-L50
[2] http://docs.openstack.org/developer/keystoneauth/migrating.html

Change-Id: I7d8f9a9f071a0d37244f8985bd7d09d49ba79eb8
Closes-Bug: 1533369

keystoneauth1/token_endpoint.py

@@ -36,3 +36,13 @@ class Token(plugin.BaseAuthPlugin):
         parameters passed to the plugin.
         """
         return self.endpoint
+
+    def get_auth_ref(self, session, **kwargs):
+        """Return the authentication reference of an auth plugin.
+
+        :param session: A session object to be used for communication
+        :type session: keystoneauth1.session.session
+        """
+        # token plugin does not have an auth ref, because it's a
+        # "static" authentication using a pre-existing token.
+        return None