HDFS-5007. Replace hard-coded property keys with DFSConfigKeys fields. Contributed by Kousuke Saruta.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1504764 13f79535-47bb-0310-9956-ffa450edef68
Merge remote-tracking branch upstream/$:{upbranch}