Cleanup CHANGES.txt for 2.1.0-beta section and make it consistent with trunk.

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HDFS-4917. Merge 1494433 from trunk.

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HDFS-4917. Revert r1494434 previous merge that included extraneous change.

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HDFS-4917. Merge 1494433 from trunk.

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Merge -c 1494476 from trunk to branch-2 to fix YARN-553. Replaced YarnClient.getNewApplication with YarnClient.createApplication which provides a directly usable ApplicationSubmissionContext to simplify the api. Contributed by Karthik Kambatla.

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