@Override
  public void nodeDeleted(String path) {
    // There is no case of visibility labels path to get deleted.
  }
A zk watcher that watches the labels table znode. This would create a znode
/hbase/visibility_labels and will have a serialized form of a set of labels in the system.
hbase-server/src/main/java/org/apache/hadoop/hbase/security/visibility/ZKVisibilityLabelWatcher.java