make postgresql jobs non voting

postgresql is currently not running in devstack gate, which means
we've got a chicken vs. egg problem in getting it running again.

Turn it to non voting so we can get it running again and that someone
could fix nova to make it work.

Change-Id: I095de9370d0007539a8ea417372205ef69187bb4
Reviewed-on: https://review.openstack.org/27367
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins