With this change, the upload CGI script will start preferring uploads
hashed as sha512, but still accept md5 as a fallback.
The message emitted on fedmsg is unchanged, because doing so would break
it. We're going to fix that later though.
https://fedorahosted.org/rel-eng/ticket/5846
... as much as possible.
The point of this patch is to make it easier to move away from md5 in a
subsequent patch, without having one monster change which would be
impossible to review.
Some md5 stuff remains hardcoded, because changing it at this point
would break compatibility.
https://fedorahosted.org/rel-eng/ticket/5846
This will make it easier to move away from md5 for the source tarballs.
It shouldn't cause any problem anyway, as Fedora runs this on EL 6 in
production, and EL 7 in staging.
