code: func (c *NodeCommand) ListActive(client auth.ClientI) error {
  	nodes, err := client.GetNodes(c.namespace, services.SkipValidation())
  	if err != nil {
  		return trace.Wrap(err)
  	}
  	coll := &serverCollection{servers: nodes}
  	coll.writeText(os.Stdout)
  	return nil
  }
docs: // ListActive retreives the list of nodes who recently sent heartbeats to
  // to a cluster and prints it to stdout