Enable mysql backend for Ceilometer

Ceilometer in icehouse fully supports mysql as
a backend and devstack defaults to mysql instead
of mongo now. This commit is an attempt to move
us to mysql for ceilometer. Commenting out
mongo bindings and leaving them in place for
now.

Change-Id: I3b4325c64d72108b791bb1bc55f0cd5f12cb8714
Implements: blueprint ceilometer-mysql-support
Merge "Enable mysql backend for Ceilometer"