Unmounts the sftp system if it's currently mounted.

def unmount(self):
        """Unmounts the sftp system if it's currently mounted."""
        if not self.mounted:
            return

        # Try to unmount properly.
        cmd = 'fusermount -u %s' % self.mount_point_local
        shell_exec(cmd)

        # The filesystem is probably still in use.
        # kill sshfs and re-run this same command (which will work then).
        if self.mounted:
            self._kill()
            shell_exec(cmd)

        self._mount_point_local_delete()