2d884bfe11
A recent change a43944cc83 have
make tempest run on liberty when liberty doesn't have tempest tests.
For example, we can see that gate-aodh-dsvm-tempest-plugin-mongodb run
https://review.openstack.org/#/c/373363/ when it should not.
This change fixes that
Change-Id: Ife55effbcc7741ef2c42c3a82e9c97be74718b53