Merge r1580156 from branch-2. YARN-1776. Fixed DelegationToken renewal to survive RM failover. Contributed by Zhijie Shen

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1580157 13f79535-47bb-0310-9956-ffa450edef68