Change database poison warning to an exception
Change database poison warning to an exception

The following warning appears in the unit test logs a number of times.

    "UserWarning: This test uses methods that set internal oslo_db
state, but it does not claim to use the database. This will conflict
with the setup of tests that do use the database and cause failures
later."

Note that this warning is only emitted once per unit test worker, so new
offenders will show up in the logs each time you fix a test until they
are all gone.

This patch is intended to be merged after all the existing warnings are
addressed by other patches in this series:

  https://review.openstack.org/#/q/topic:bug/1568414

Change-Id: I91909a356187bac70fa2fa920cebf3888410ae01
Closes-Bug: #1568414

klmitch/nova,jianghuaw/nova,openstack/nova,jianghuaw/nova,hanlind/nova,openstack/nova,mikalstill/nova,klmitch/nova,gooddata/openstack-nova,mahak/nova,vmturbo/nova,jianghuaw/nova,rajalokan/nova,klmitch/nova,vmturbo/nova,rahulunair/nova,mikalstill/nova,rajalokan/nova,jianghuaw/nova,hanlind/nova,gooddata/openstack-nova,rajalokan/nova,vmturbo/nova,phenoxim/nova,openstack/nova,rajalokan/nova,Juniper/nova,mikalstill/nova,rahulunair/nova,klmitch/nova,gooddata/openstack-nova,Juniper/nova,hanlind/nova,mahak/nova,Juniper/nova,phenoxim/nova,vmturbo/nova,Juniper/nova,mahak/nova,gooddata/openstack-nova,rahulunair/nova