svn merge -c 1514156 from trunk for HDFS-4898. BlockPlacementPolicyWithNodeGroup.chooseRemoteRack() fails to properly fallback to local rack.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1514157 13f79535-47bb-0310-9956-ffa450edef68