Pass branch-override on jobs missing it

* .../files/jenkins_job_builder/config/devstack-gate.yaml: Pass the
branch-override parameter into the BRANCH_OVERRIDE envvar for
periodic-tempest-dsvm-all-.*, periodic-tempest-dsvm-coverage-.* and
periodic-tempest-dsvm-stress-.* jobs. Without this, devstack-gate
passes $BASE/new into the setup_workspace base_branch because
there's no ZUUL_BRANCH for periodic-trigger jobs. That leaves the
positional DEST argument empty and results in devstack-gate not
getting properly set up, leaving jobs to fail with a no such file or
directory error when trying to move past the setup phase.

Change-Id: I51bffcc529585cdb86876ad092eeb7353e231db9
Merge "Pass branch-override on jobs missing it"