rgw: data log contains bucket instance info in key

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: add a system request param to select op bucket instance

Added rgwx-bucket-instance request param that selects the actual bucket
instance to use for this request.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>