svn merge -c 1611750 from trunk for HDFS-6616. Add exclude-datanodes feature to WebHDFS redirection so that it will not redirect retries to the same datanode.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1611751 13f79535-47bb-0310-9956-ffa450edef68