Before this fix rclone would just use the authorised bucket regardless
of what bucket you put on the command line.
This uses the new `bucketName` response in the API and checks that the
user is using the correct bucket name to avoid accidents.
Fixes#2839
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
