fix nova auth configuration

Based on
http://lists.openstack.org/pipermail/openstack/2014-January/004667.html,
there is confusion in the openstack community on how auth_uri in
nova.conf is supposed to interact with the other auth_* settings.

I've modified our config to use both for now.

Change-Id: I1433305e23d872cd773599c85053ba8f9aeb6145