Factor out _update_db_object method in handlers

The update method is now moved to the db object.
That way, if solum is accessed by something else than the
handlers, it still works fine

Closes bug 1282616
https://bugs.launchpad.net/solum/+bug/1282616

Change-Id: I90430588c666a1d5c8ea6801c92facb059f47296
Merge "Factor out _update_db_object method in handlers"