HDFS-4269. DatanodeManager#registerDatanode rejects all datanode registrations
from localhost in single-node developer setup  (Contributed by Chris Nauroth)


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1449256 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-HADOOP-7435-branch-2-gd