rgw: add compatibility for MultipartUpload

Fixes: #6677
It seems that there's some external library that uses MultipartUpload
instead of CompleteMultipartUpload, even though it's not part of the
documented api. Support it.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1259 from ceph/wip-6677

rgw: add compatibility for MultipartUpload

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>