HADOOP-10274 Lower the logging level from ERROR to WARN for UGI.doAs method (Takeshi Miao via stack)

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HDFS-5833. Merging r1561938 from trunk to branch-2

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HADOOP-9830. Merging r1561951 from trunk to branch-2

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HADOOP-10255. Merge 1561959 and 1561961 from trunk.

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HADOOP-10292. Restore HttpServer from branch-2.2 in branch-2. Contributed by Haohui Mai.

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YARN-1642. RMDTRenewer#getRMClient should use ClientRMProxy (kasha)

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YARN-1629. IndexOutOfBoundsException in MaxRunningAppsEnforcer (Sandy Ryza)

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