Only run Barbican KMIP gate on master branch
Only run Barbican KMIP gate on master branch

The KMIP gate only works for the current master and
for future releases. It should be disabled for the
current stable branches.

Change-Id: I80850a2452723493d746f6c3ac8de34876c60c9e

openstack-infra/project-config,openstack-infra/project-config