Wrap disk file construction in a method; pep8

Change-Id: I5e7af267bcbb510108f62a9c66b9baa6771cc8fe
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Reviewed-on: http://review.gluster.org/5985
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>