YARN-581. Added a test to verify that app delegation tokens are restored after RM restart. Contributed by Jian He.
svn merge --ignore-ancestry -c 1471187 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1471188 13f79535-47bb-0310-9956-ffa450edef68