Add new command to run dsvm-gate-tests

This command is added to run the devstack VM gate tests under
OpenStack CI. The entrypoint will be a devstack-vm-gate setup.
The script handles setting up test configuration through kick-start,
and actually running the Trove tests.

Change-Id: I78471d7f11d0f1a23e2efc28da2e62c5707e575a