Merge 1563640 from trunk: HDFS-5828. BlockPlacementPolicyWithNodeGroup can place multiple replicas on the same node group when dfs.namenode.avoid.write.stale.datanode is true. (Buddy via junping_du)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1563643 13f79535-47bb-0310-9956-ffa450edef68