HADOOP-10208. Merge change r1556602 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1556604 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10207. TestUserGroupInformation#testLogin is flaky (jxiang via cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1556670 13f79535-47bb-0310-9956-ffa450edef68