Rollback configuration to filename or to self.rollback_cfg file.

def rollback(self):
        """Rollback configuration to filename or to self.rollback_cfg file."""
        filename = self.rollback_cfg
        cfg_file = self._gen_full_path(filename)
        if not self._check_file_exists(cfg_file):
            raise ReplaceConfigException("Rollback config file does not exist")
        cmd = "configure replace {} force".format(cfg_file)
        self.device.send_command_expect(cmd)

        # After a rollback - we no longer know whether this is configured or not.
        self.prompt_quiet_configured = None

        # Save config to startup
        self.device.save_config()