Merging r1541995 from trunk to branch-2 for YARN-1222. Make improvements in ZKRMStateStore for fencing (Karthik Kambatla via bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1542026 13f79535-47bb-0310-9956-ffa450edef68