code: def slave_envs(self):
          
          if self.hostIP == 'dns':
              host = socket.gethostname()
          elif self.hostIP == 'ip':
              host = socket.gethostbyname(socket.getfqdn())
          else:
              host = self.hostIP
          return {'rabit_tracker_uri': host,
                  'rabit_tracker_port': self.port}
docs: get enviroment variables for slaves
          can be passed in as args or envs