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
HDFS-4278. Log an ERROR when DFS_BLOCK_ACCESS_TOKEN_ENABLE config is disabled but security is turned on. Contributed by Kousuke Saruta. (harsh)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1504784 13f79535-47bb-0310-9956-ffa450edef68