HDFS-2576. Enhances the DistributedFileSystem's create API so that clients can specify favored datanodes for a file's blocks. Contributed by Devaraj Das and Pritam Damania.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1476395 13f79535-47bb-0310-9956-ffa450edef68