HDFS-4912. Merge 1502634 from trunk

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1502640 13f79535-47bb-0310-9956-ffa450edef68
HDFS-4982. JournalNode should relogin from keytab before fetching logs from other JNs. Contributed by Todd Lipcon.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1502683 13f79535-47bb-0310-9956-ffa450edef68