Randomize file list in SimpleCopyListing

When copying files to S3, based on file listing some mappers can get into S3 partition hotspots. This would be more visible, when data is copied from hive warehouse with lots of partitions (e.g date partitions). In such cases, some of the tasks would tend to be a lot more slower than others. It would be good to randomize the file paths which are written out in SimpleCopyListing to avoid this issue.