core: path in bucket for s3 coordinator

[BorisTyshkevich]

Now, s3 coordinator creates directories in the root of the provided bucket.
It's not good when a bucket is used for something else.
It is better to have a parameter to specify a path or allow a path at the end of the bucket:

trcli replicate --coordinator s3 --coordinator-s3-bucket XXXX --coordinator-s3-path YYYY
trcli replicate --coordinator s3 --coordinator-s3-bucket XXXX/YYYY

[laskoviymishka]

That's a good improvement.
Relatevely easy to implement, need to pass it here and handle there.