code: func WriteHostUUID(dataDir string, id string) error {
  	err := ioutil.WriteFile(filepath.Join(dataDir, HostUUIDFile), []byte(id), os.ModeExclusive|0400)
  	if err != nil {
  		return trace.ConvertSystemError(err)
  	}
  	return nil
  }
docs: // WriteHostUUID writes host UUID into a file