Use OVERRIDE_ZUUL_BRANCH in devstack-vm-gate.sh

When a change on master branch triggered a stable job,
the stable job's localrc configured according to the
master rules instead of the stable rules.

For example: The stable/havana job on the devstack-gate changes
attempted to install trove, which was not part of the integrated
set at the havana time.
It is also caused failures on the stable/havana postgresql jobs.

We saw similar issues on the periodic jobs as well.

The issue can be solved by using the OVERRIDE_ZUUL_BRANCH, instead of
ZUUL_BRANCH.

devstack-vm-gate-wrap.sh always defines the OVERRIDE_ZUUL_BRANCH.

Change-Id: Id49f9730a2feeca51dee057d5aec6968a9f62bb3
fix trove merge

trove service enablement is missing a comma. this causes trove
service and previous service to merge into one non-existent service

Change-Id: Ia487bce09d2f3a6b352c772597063269f8320457
Merge "fix trove merge"
Merge "Use OVERRIDE_ZUUL_BRANCH in devstack-vm-gate.sh"