fix postgres user dependency

Change-Id: Ia3a661f46c52a412363892b79051600063447931
Reviewed-on: https://review.openstack.org/21033
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins