HADOOP-9361: Strictly define FileSystem APIs

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HADOOP-9361: Strictly define FileSystem APIs - HDFS portion

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HADOOP-9361: Strictly define FileSystem APIs - OpenStack portion

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HADOOP-9361: site and gitignore

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HADOOP-9361: changes.txt

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HADOOP-10312 changes.txt updated in wrong place

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Merge r1607655 from trunk: YARN-2242. Improve exception information on AM launch crashes. (Contributed by Li Lu)

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