svn merge -c 1523875 from trunk for HDFS-5207. In BlockPlacementPolicy.chooseTarget(..), change the writer and the excludedNodes parameter types respectively to Node and Set.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1523877 13f79535-47bb-0310-9956-ffa450edef68