YARN-829. Renamed RMTokenSelector to be RMDelegationTokenSelector. Contributed by Zhijie Shen.
svn merge --ignore-ancestry -c 1493577 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1493578 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-HADOOP-9166-branch-2-gd