Remove stable/liberty pin
Remove stable/liberty pin

The nova revert[1] has landed, so we should be able to remove the pin
and use the latest stable/liberty delorean repo.

[1] https://review.openstack.org/#/c/264793/

Change-Id: I3c1ecadceb9623fa03edaf87226297834448d161

openstack/tripleo-common,openstack/tripleo-common