Merge r1490473 from branch-2 to branch-2.1-beta for YARN-759. Create Command enum in AllocateResponse (bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1490475 13f79535-47bb-0310-9956-ffa450edef68
svn merge -c 1490486 Merging from trunk to branch-2.1.0-beta to fix HDFS-4862.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1490488 13f79535-47bb-0310-9956-ffa450edef68