HDFS-5544. Adding Test case For Checking dfs.checksum.type as NULL value. Contributed by Sathish.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1544596 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5552. Fix wrong information of Cluster summay in dfshealth.html. Contributed by Haohui Mai.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1544627 13f79535-47bb-0310-9956-ffa450edef68
Move HDFS-5444 and HDFS-5525 to branch 2.3.0 section in CHANGES.txt

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1544631 13f79535-47bb-0310-9956-ffa450edef68