Switch Ceilometer default backend to MySQL

MongoDB 2.4 not being available in Ubuntu cloud archive for a while now,
and the catch up done by this driver allows me to think it might be a
good idea to switch by default on SQL for now on devstack.

We can add another job to have Ceilometer tested on MongoDB too later.

Change-Id: I74c3c436d009fed898c5ae4ffb82763e9a337d90
diff --git a/lib/ceilometer b/lib/ceilometer
index 8768122..46e4624 100644
--- a/lib/ceilometer
+++ b/lib/ceilometer
@@ -43,7 +43,7 @@
 CEILOMETER_BIN_DIR=$(get_python_exec_prefix)
 
 # Set up database backend
-CEILOMETER_BACKEND=${CEILOMETER_BACKEND:-mongodb}
+CEILOMETER_BACKEND=${CEILOMETER_BACKEND:-mysql}
 
 # Functions
 # ---------