gate on postgresql

now that keystone and nova bugs on postgresql have been fixed,
and pg runs are constistently green, turn on pg gating, to prevent
bugs from slipping back in.

Change-Id: I1fbed41393b9908b602739be56839aa7959b07e8
Reviewed-on: https://review.openstack.org/21071
Reviewed-by: Davanum Srinivas (DIMS) <davanum@gmail.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Kurt Taylor <krtaylor@us.ibm.com>
Reviewed-by: Paul Belanger <paul.belanger@polybeacon.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins