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