Fix postgresql test failure
Fix postgresql test failure

See the failure here:
http://logs.openstack.org/95/329895/1/check/gate-mistral-tox-db-unit-postgresql/44f8223/console.html

Change-Id: I65092c6e38c86c6395b5ac9107173784902e4bde

openstack/mistral,StackStorm/mistral,StackStorm/mistral,openstack/mistral