lookaside: Remove tempfile when hash verification fails

Without this, tempfiles just accumulate.
lookaside: Drop EL < 6 support

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.
lookaside: De-hardcode md5 assumptions...

... 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
lookaside: Add support for sha512 file hashes

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
lookaside: Add the hash type to the folder structure

This makes it more explicit which hash was used for a given tarball.

https://fedorahosted.org/rel-eng/ticket/5846