Currently, devstack only support mongodb as ceilometer storage backend,
this patch is to add mysql storage support for ceilometer.
If you want to use mysql as backend during developing, you can specify
CEILOMETER_BACKEND=mysql in localrc file.
If you use mongodb, just ignore the parameter.
Change-Id: Ic2f475a9baa6d71a43cd29a6ca777ac972e47b0a
Implements: blueprint ceilometer-mysql-support