Switch heat to use branch-tarball job.

Tag based versioning has landed in heat, it should run the same
tarball jobs as the rest.

Change-Id: I313e887eb4abeb9f4826ab07a64d896993275503
Reviewed-on: https://review.openstack.org/22447
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Thierry Carrez <thierry@openstack.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins