ensure auto skipping of db tests is verboten

we have infrastructure to prevent skipping the database tests in
nova, but apparently we never turned on the CI side of this
equation.

Change-Id: I4d3a87256e6ea5e59fabbda0cde60c2e415b4f1f
Reviewed-on: https://review.openstack.org/22464
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins