svn merge -c 1615015 from trunk for HDFS-6685. Balancer should preserve storage type of replicas.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1615016 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6797: Merging r1615017 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1615018 13f79535-47bb-0310-9956-ffa450edef68