HDFS-5007. Merge change r1504764 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1504765 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/branches/branch-2@1504790 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-YAHOO-218-branch-2-gd