rgw, radosgw-admin: bucket link uses bucket instance id now

Fixes: 7499
We need to link user to a specific bucket instance. This updates both
the radosgw-admin link command, and the RESTful admin api.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1653 from ceph/wip-7499

rgw, radosgw-admin: bucket link uses bucket instance id now

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>