Merge r1613515 from trunk. YARN-2211. Persist AMRMToken master key in RMStateStore for RM recovery. Contributed by Xuan Gong

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1613516 13f79535-47bb-0310-9956-ffa450edef68