HDFS-6722. Merge r1616669 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1616670 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10771. Refactor HTTP delegation support out of httpfs to common, PART 1. (tucu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1616673 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10771. Refactor HTTP delegation support out of httpfs to common, PART 2. (tucu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1616674 13f79535-47bb-0310-9956-ffa450edef68