svn merge -c 1565505 merging from trunk to branch-2 to fix:HDFS-5807. TestBalancerWithNodeGroup.testBalancerWithNodeGroup fails intermittently.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1565506 13f79535-47bb-0310-9956-ffa450edef68