HDFS-573. Merging change r1616814 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1616817 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6823. dfs.web.authentication.kerberos.principal shows up in logs for insecure HDFS

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1616847 13f79535-47bb-0310-9956-ffa450edef68