Fix neutron func job to use gate-specific tox env

The neutron tox config now has a gate-specific tox env (tox -e
dsvm-functional) that configures the environment variables necessary
to run tests that require the use of sudo+rootwrap.

Change-Id: I25b50e3c0ddb8714387e0e3902c237a0a41a3443
Closes-bug: #1336172
Merge "Fix neutron func job to use gate-specific tox env"