Unmounts and cleans-up after a previous mount().

def unmount(self, cid):
        """
        Unmounts and cleans-up after a previous mount().
        """
        driver = self.client.info()['Driver']
        driver_unmount_fn = getattr(self, "_unmount_" + driver,
                                    self._unsupported_backend)
        driver_unmount_fn(cid)