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>
(cherry picked from commit ddc2e1a8e39a5c6b9b224c3eebd1c0e762ca5782)