Merge r1580077 from trunk. YARN-1849. Fixed NPE in ResourceTrackerService#registerNodeManager for UAM. Contributed by Karthik Kambatla

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1580078 13f79535-47bb-0310-9956-ffa450edef68