Merge r1490440 from branch-2 to branch-2.1-beta for YARN-761. TestNMClientAsync fails sometimes (Zhijie Shen via bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1490444 13f79535-47bb-0310-9956-ffa450edef68