Allow db fields configuration without doing the dbsync

Allow db fields configuration without doing the dbsync

This commit allows one to configure the db fields without running the actual
dbsync command. It remains compliant with the current implementation, leaving
exec {'ceilometer-dbsync': in the catalog. One, now, can decide if it wants
the dbsync to take place or not.

This is useful when one picks mongodb as a backend and wants to use replicaset
This way it can scale horizontally and painlessly using the initSync feature
of mongodb. With this pattern one can add new member of the replica set
easily.

Change-Id: I8618bb385147176512fbda1540ba75ac95fccbf1