add 2 features in s3 output plugin

[cnhappier]

1. make codec configurable
2. move file to bucket in graceful shutdown

[dmlogs]

1. is covered in logstash/outputs/base

2. I would argue should be a configuration. Forcing a flush may not always be desirable.

[cnhappier]

according to dmlogs's comment, add move_file_to_bucket flag to disable the move file in shutdown

[elasticsearch-release]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run; then say 'jenkins, test it'.

[runningman84]

@cnhappier can you sign the CLA in order to get this integrated?

[cnhappier]

@karmi @runningman84 I've signed the CLA, how can I continue to merge this?

[chadbean]

@cnhappier looks like there's a merge conflict now. If that gets fixed is this in a good place to merge? This would be very useful. Another thought: what if this was updated to upload all files in the persisted queue to S3?