svn merge -c 1612493 from trunk for HDFS-6702. Change DFSClient to pass the StorageType from the namenode to datanodes and change datanode to write block replicas using the specified storage type.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1612494 13f79535-47bb-0310-9956-ffa450edef68