Fix functional job for Ceilometer

Change-Id: I23a00a1c98016ff3c66f60e2eb153d8a5a5a28d0

jenkins/jobs/ceilometer.yaml

@@ -77,6 +77,7 @@
           #!/bin/bash -xe
           export PYTHONUNBUFFERED=true
           export DEVSTACK_GATE_TIMEOUT=120
+          export DEVSTACK_GATE_CEILOMETER_BACKEND={backend}
           export BRANCH_OVERRIDE={branch-override}
           if [ "$BRANCH_OVERRIDE" != "default" ] ; then
               export OVERRIDE_ZUUL_BRANCH=$BRANCH_OVERRIDE