HDFS-5941. Merging change r1568019 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1568021 13f79535-47bb-0310-9956-ffa450edef68
YARN-1676. Modified RM HA handling of user-to-group mappings to be available across RM failover by making using of a remote configuration-provider. Contributed by Xuan Gong.
svn merge --ignore-ancestry -c 1568041 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1568042 13f79535-47bb-0310-9956-ffa450edef68