Add a delete() method to DiskFile

In preparation for making the DiskFile class an abstraction layer for
the backend to all the object server REST API services, we add a
simple delete method which encapsulates the required behavior.

Change-Id: I539ad53c297e4c572e2f93c2524188fb89b2db40
Signed-off-by: Peter Portante <peter.portante@redhat.com>