code: String processVersion(String version) {
      if (version == null || version.trim().isEmpty() || version.equals(APPENGINE_CONFIG)) {
        throw new IllegalArgumentException(VERSION_ERROR);
      } else if (version.equals(GCLOUD_CONFIG)) {
        return null;
      }
      return version;
    }
docs: Process user configuration of "version". If set to GCLOUD_CONFIG then allow gcloud to generate
  a version. If set but not a keyword then just return the set value.