merge -c 1459040 from trunk to branch-2 to fix YARN-396. Rationalize AllocateResponse in RM Scheduler API. Contributed by Zhijie Shen.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1459044 13f79535-47bb-0310-9956-ffa450edef68