code: private static String lookupStorageServiceByApp(RESTClient restClient, String applicationName) {
          Utils.require(applicationName != null, "Missing application name");
          if (applicationName != null) {
              ApplicationDefinition appDef =  Command.getAppDef(restClient, applicationName);
              Utils.require(appDef != null, "Unknown application: %s", applicationName);
              return appDef.getStorageService();
          }
          return null;
      }
docs: Convenient method to lookup storageService of the application
  @param restClient
  @param applicationName
  @return storage service name