public void updateExistingConfigs(AddServiceInfo request, Set<String> existingServices) {
    LOG.info("Updating existing configurations for {}", request);
    Set<ClusterRequest> requests = createConfigRequestsForExistingServices(request, existingServices);
    updateCluster(request, requests, "Error updating configurations for %s");
  }
@return {@code Optional} with the cluster's kerberos descriptor artifact if it exists, otherwise empty {@code Optional}
ambari-server/src/main/java/org/apache/ambari/server/topology/addservice/ResourceProviderAdapter.java