Factor duplicate code in __deepcopy__ methods into one method (#1095244)

Related: fed#1095244

The method treats attributes in omit and attributes w/ value None the
same way.  Previously, an attribute with a None value was not handled
specially, which could lead to "NoneType has no attribute <x>"
sorts of errors.

Signed-off-by: mulhern <amulhern@redhat.com>