Merge r1595145 from trunk: HDFS-6250. Fix test failed in TestBalancerWithNodeGroup.testBalancerWithRackLocality (Contributed by Binglin Chang and Chen He

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1595146 13f79535-47bb-0310-9956-ffa450edef68