code: def workers(cmd):
      
      if config.dbserver.multi_user and getpass.getuser() != 'openquake':
          sys.exit('oq workers only works in single user mode')
  
      master = workerpool.WorkerMaster(config.dbserver.host,
                                       **config.zworkers)
      print(getattr(master, cmd)())
docs: start/stop/restart the workers, or return their status