Fix multipart upload is required SLO in pipeline

Multipart upload is enable by default, but it needs "slo" middleware
support. If it isn't exist in pipeline, multipart upload still work, but
download is not work(that's only xml file you can get.)

If there is no "slo" in pipeline, all requests of multipart from client
will be reject(return S3NotImplemented, 501 Not Implemented).

Change-Id: If0a862d9f80cc806f5b0a8a0e87c70b17920f635
Closes-Bug: #1396039