config: use .pgpass for db password

database connection parameters read from .pgpass were
all passed to the config parser as defaults with
the exception of the pg_pass default.
This patch set pg_pass with the value read from .pgpass
if not specified by command line or config file.

Change-Id: I60adef202e00cf9a49f9b512e499064ae705a4f1
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>