Merge 1524829 from trunk to branch-2 for YARN-353. Add Zookeeper-based store implementation for RMStateStore. Contributed by Bikas Saha, Jian He and Karthik Kambatla.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1524831 13f79535-47bb-0310-9956-ffa450edef68