Renames DbClient to CommonDbClient to avoid confusion

POL comunicates with two different databases, a relational
one, that stores records for alarms, scaling groups and more,
and a nonrelational one, that is common to all OSM modules,
which we call CommonDB, that currently corrsponds to a MongoDB
instance and stores nsds, vnfds, etc.
The name db_client and DbClient used in the file and
class that comunicates with CommonDB may be open to confussion,
so it has been renamed as common_db_client and CommonDbClient.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: I8209cbc23b5ab129221d1c928703929e4f405a4c