rgw: calculate user manifest

Fixes: #8169
Backport: firefly
We didn't calculate the user manifest's object etag at all. The etag
needs to be the md5 of the contantenation of all the parts' etags.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1772 from ceph/wip-8169

rgw: calculate user manifest

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>