openstack-origin-git currently only supports YAML that specifies
the git repositories to deploy from.
This adds support for default openstack-origin-git values. The
default values supported are: icehouse, kilo, liberty, mitaka,
and master. For example: openstack-origin-git=master.
Change-Id: Ib2e0224f70e51d2686b6bb1f28e6ad528621eafb