Enable overriding of Ceilometer storage backend

That is needed to have gate jobs running on different backends.

Change-Id: Ib06f91d05ca3d8eb703b55dbac309fa5d0df7d49

devstack-vm-gate-wrap.sh

@@ -280,6 +280,10 @@ export DEVSTACK_GATE_TIMEOUT=${DEVSTACK_GATE_TIMEOUT:-60}
 # single-branch checkouts; not used for grenade)
 export OVERRIDE_ZUUL_BRANCH=${OVERRIDE_ZUUL_BRANCH:-$ZUUL_BRANCH}
 
+# Set Ceilometer backend to override the default one. It could be mysql,
+# postgresql, mongodb.
+export DEVSTACK_GATE_CEILOMETER_BACKEND=${DEVSTACK_GATE_CEILOMETER_BACKEND:-mysql}
+
 if ! function_exists "gate_hook"; then
   # the command we use to run the gate
   function gate_hook {

devstack-vm-gate.sh

@@ -91,6 +91,7 @@ USE_GET_PIP=1
 UNDO_REQUIREMENTS=False
 CINDER_PERIODIC_INTERVAL=10
 export OS_NO_CACHE=True
+CEILOMETER_BACKEND=$DEVSTACK_GATE_CEILOMETER_BACKEND
 EOF
 
     if [[ "$DEVSTACK_CINDER_SECURE_DELETE" -eq "0" ]]; then