HDFS-6722. Merge r1616669 from trunk.

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HADOOP-10771. Refactor HTTP delegation support out of httpfs to common, PART 1. (tucu)

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HADOOP-10771. Refactor HTTP delegation support out of httpfs to common, PART 2. (tucu)

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HDFS-6772: Merging r1616680 from trunk to branch-2.

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HADOOP-10771. Addendum, default realm correction to common test krb5.conf file. (tucu)

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