Comment 4 for bug 1736252

The fix here isn't to disable (explicitly) mistral tests from the job definition, but to stop installing tempest test plugins (like https://github.com/openstack/mistral/tree/master/mistral_tempest_tests)

add nova and neutron tempest plugins to default config
https://review.openstack.org/#/c/525657

stop installing plugins in convert image
https://review.openstack.org/#/c/525659/