svn merge -c 1532468 from trunk for HDFS-5338. Add a conf to disable hostname check in datanode registration.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1532473 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-100046. Print a log message when SSL is enabled (David S. Wang via wang)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1532479 13f79535-47bb-0310-9956-ffa450edef68