Dedupe getNodePath implementation in ZKRMStateStore to use ZKCuratorManager

ZKRMStateStore implements getNodePath like this:

  @VisibleForTesting
  String getNodePath(String root, String nodeName) {
    return (root + "/" + nodeName);
  }

In HADOOP-14741 ZKCuratorManager adds the same implementation.

We should dedupe this.