Merge r1543707 from trunk to branch-2 for YARN-744. Race condition in ApplicationMasterService.allocate .. It might process same allocate request twice resulting in additional containers getting allocated. (Omkar Vinit Joshi via bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1543709 13f79535-47bb-0310-9956-ffa450edef68