HDFS-4794. Browsing filesystem via webui throws kerberos exception when NN service RPC is enabled in a secure cluster. Contribued by Benoy Antony.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1@1531794 13f79535-47bb-0310-9956-ffa450edef68