Add stable jobs to the gate

Since neutron-tempest-plugin is branchless, it is necessary to gate
againest all stable branches to avoid breakage in stable branches.

Related-Bug: #1765008
Change-Id: Ib175678c0a655568054a4c2d595b890ff85e00ec
diff --git a/playbooks/neutron-tempest-plugin-api/run.yaml b/playbooks/neutron-tempest-plugin-api/run.yaml
index 12638cc..230ac10 100644
--- a/playbooks/neutron-tempest-plugin-api/run.yaml
+++ b/playbooks/neutron-tempest-plugin-api/run.yaml
@@ -34,7 +34,7 @@
           export DEVSTACK_GATE_EXERCISES=0
           export DEVSTACK_GATE_TEMPEST_REGEX="neutron_tempest_plugin.api"
           export DEVSTACK_LOCAL_CONFIG="enable_plugin neutron-tempest-plugin git://git.openstack.org/openstack/neutron-tempest-plugin"
-          export BRANCH_OVERRIDE=default
+          export BRANCH_OVERRIDE="{{ branch_override | default('default') }}"
           if [ "$BRANCH_OVERRIDE" != "default" ] ; then
               export OVERRIDE_ZUUL_BRANCH=$BRANCH_OVERRIDE
           fi