Cache auth_ref from keystoneclient

Auth_ref is documented as a serializable object which we can save off
and use to reinitialize keystoneclient. That should in turn be passable
to heatclient directly. With this code, we will avoid authorizing with
Heat if we have a token in the cache.

Change-Id: I3dc6c3726054e77449cc9be46229e6d3521d4ef2
Closes-Bug: #1321437