HADOOP-10929. Merging change r1616616 from trunk

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1616618 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6728. Dynamically add new volumes to DataStorage, formatted if necessary. Contributed by Lei Xu.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1616620 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6740. Make FSDataset support adding data volumes dynamically. Contributed by Lei Xu.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1616624 13f79535-47bb-0310-9956-ffa450edef68