Refactor DiskDir and DiskAccount to use DiskCommon

We have refactored DiskDir and DiskAccount so that truly common,
shared functions live in the base class DiskCommon, and each of these
then sub-class that. This allows us to isolate unique behaviors.

We have also enabled all the skipped unit tests in test_diskdir.py
that we could, removing a few that we don't need right now.

Change-Id: I48d9c915108df8cc92c3e9a764563e1d10c50050
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Reviewed-on: http://review.gluster.org/5148
Reviewed-by: Mohammed Junaid <junaid@redhat.com>
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>