Add DiskDir unit test skeleton and pep8 filter

The new DiskDir unit test skeleton is quite incomplete, but gets the
DiskDir module on the board for modules covered, explicitly exposing
the fact that we need to get test coverage. This is a first step.

At the same time, we also update all the modules we have applying the
fix for pep8 errors now run under tox. We can then add a Jenkins
pre-commit job to fail on pep8 errors. This brings our code to parity
with what they are doing in OpenStack Swift.

Change-Id: Ia0565606512efda6e73f67bd00269177b89db858
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Reviewed-on: http://review.gluster.org/5080
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>