Handle requests without host headers / by IP

Hi,

We deliberately don't support connections without a Host header so people don't hardcode IPs of the fastest mirror at the time and then complain when that IP changed roles[1].

With the charm now supporting multiple roles on the same host (archive, releases, security), we'll need to figure out a better way to do this. Perhaps an empty page or redirect to the main ubuntu.com download page? I think we can do this with a default vhost in each of the generated vhost config.

[1]https://code.launchpad.net/~hloeung/canonical-is-charms/ubuntu-mirror-default-vhost-and-server-status/+merge/254717