HADOOP-16357. TeraSort Job failing on S3 DirectoryStagingCommitter: destination path exists. Contributed by Steve Loughran.
* changes the default for the staging committer to append, as we get for the classic FileOutputFormat committer * adds a check for the dest path being a file not a dir * adds tests for this * Changes AbstractCommitTerasortIT. to not use the simple parser, so fails if the file is present.