Creation of ceilometer::db::sync

Creation of ceilometer::db::sync

In order to standardize the way dbsync are run across our modules,
we create a new class ceilometer::db::sync.
This class will be included if sync_db is enabled.

By making this transition the ceilometer::db::sync can be returned
by the ENC.

A use case would be in an highly available environment, with 3 galera
nodes, include ceilometer::db on every node with sync_db set to false
and have the ENC return ceilometer::db::sync just for one node.

Change-Id: I410f41fb4e61be848372820cdeb2110efc327161