Make gate-tempest-devstack-vm-large-ops voting

This test has been running for about a week now as non-voting so
hopefully developers should be used to it. And the job has been running
successfully for a week with no known false positives. So time to gate!

Change-Id: Id725927dd76f5f88261ea95ed3a93dec9d9202ef
Merge "Make gate-tempest-devstack-vm-large-ops voting"