Unmounts the specified sftp system.
        Usage: sftpman umount {id}..

def command_umount(self, system_id, *system_ids):
        """Unmounts the specified sftp system.
        Usage: sftpman umount {id}..
        """
        system_ids = (system_id,) + system_ids
        has_failed = False
        for system_id in system_ids:
            try:
                system = SystemModel.create_by_id(system_id, self.environment)
                controller = SystemControllerModel(system, self.environment)
                controller.unmount()
            except SftpConfigException as e:
                sys.stderr.write('Cannot unmount %s: %s\n\n' % (system_id, str(e)))
                has_failed = True
        if has_failed:
            sys.exit(1)