Make gate-solum-devstack-dsvm voting

This job appears to be working as expected now, so make it voting.
It currently does the following:

 - Uses devstack-gate to:
   - run devstack, including the devstack support for solum
   - run the functional tests included with solum against the stack

Change-Id: Ia09a8da94914976f8e9ee3a7a318a868ad56ee72
Merge "Make gate-solum-devstack-dsvm voting"