Prefix the updateinfo file with its hash

All yum repository metadata files are prefixed by their hash. (except `repomd.xml`, for obvious reasons)

For example:
```
# sha256sum *
26ccf3fcca97e8dc2f8a3ce6d4b2f49e482b1d8d51cdf93438cdc08e3d4b6e4a  26ccf3fcca97e8dc2f8a3ce6d4b2f49e482b1d8d51cdf93438cdc08e3d4b6e4a-primary.xml.gz
```

However, the updateinfo one is not, which is a bit of an eyesore. :wink: 

It's more than just cosmetics and consistency, though, this is causing us a real issue.

At work, we use the Amazon CDN to implement mirroring of our Yum repositories. All files are cached on the CDN for the default duration (i think it's 24 hours), except `repomd.xml` which is never cached.

This works great for RPMS (they never change once published) and all the metadata files (their names are prefixed with their hash, and as such the CDN sees them as different files).

However, we keep running into trouble with `updateinfo.xml.gz`: the CDN keeps distributing the old file even though a new one was generated.

Because the updateinfo metadata can be quite big, we'd rather prefix its name with a hash (like other Yum repository metadata), and let the CDN handle it, rather than telling the CDN to never cache it, which would put more load on our primary repository server.