object-storage: pyflakes for utils and DiskFile

Apply a bunch of code cleanup based on running pyflakes for utils and
DiskFile modules (also removing an unnecessary return statement in
Gluster_DiskFile.put() method.

Change-Id: Ia11fed7c574f5326415a716ddbb16c3255c16d9a
Signed-off-by: Peter Portante <peter.portante@redhat.com>
object-storage: Refactor and add unit tests for DiskDir

Change-Id: Iafc89a1ce069532cbd07e6ff97071d1bce0a0faa
Signed-off-by: Peter Portante <peter.portante@redhat.com>