Allow the neutron functional job to gate merges

The neutron functional job (neutron-dsvm-functional) has been passing
consistently as a non-voting check job.  It needs to be allowed to
vote and gate merges for functional test additions to be useful.

Implements: blueprint neutron-testing-refactor

Change-Id: I296570049c84472228ea2b8c776a58cb3c50b901
Merge "Allow the neutron functional job to gate merges"