Bug 4560: Improve config system logging for debuggability

Manually cherry-picked from
https://git.opendaylight.org/gerrit/#/c/28985 as the files have moved in
master.

Also the code has changed slightly in master, specifically the
ConfigPusherImplTest no longer uses a Thread uncaught exception handler
for verification. However it does rely on exceptions thrown from the
ConfigPusherImpl so, to keep the same behavior, I added a
propagateExceptions flag to ConfigPusherImpl#process. The
ConfigPersisterActivator production code passes false so unchecked
exceptions aren't handled as uncaught exceptions.

Change-Id: Iabc22030abc22cf11a1476986ba3d3366021b4fb
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>