CEPH uses a special bucket form `tenant:bucket` for multitentant
access using S3 as documented here:
https://docs.ceph.com/en/reef/radosgw/multitenancy/#s3
However when doing multipart uploads, in the reply from
`CreateMultipart` the `tenant:` was missing from the `Bucket` response
rclone was using to build the `UploadPart` request. This caused a 404
failure return. This may be a CEPH bug, but it is easy to work around.
This changes the code to use the `Bucket` and `Key` that we used in
`CreateMultipart` in `UploadPart` rather than the one returned from
`CreateMultipart` which fixes the problem.
See: https://forum.rclone.org/t/rclone-zcat-does-not-work-with-a-multitenant-ceph-backend/48618