Follow-up: VyOS builds and HTTPS

We've made HTTP on the host optional, and restored the real directory index (provided by the Apache HTTP's mod_autoindex) instead of using the DirectoryLister that was proven a bit problematic with APT.

Since we had to change the default repository URL anyway, I also took a chance to finally make it configurable rather than hardcoded (T519). Now you can specify a custom URL with the --vyos-mirror="$URL" option. It defaults to the plain HTTP URL right now for the reason stated below.

I have also found a way to make live-build include apt-transport-https packages at the bootstrap stage and enable it to use HTTPS servers for building images. However, for some reason it doesn't work for me, apt says it cannot fetch the package index, while fetching that file with curl works just fine from the same host. I'm not sure what the issue may be. If you verify that it works for you or doesn't, or you know how to make it work, please comment upon T422.

Actual builds "still" don't work but for a completely unrelated reasons: mdns-repeater package needed for the recently merged mDNS repeater feature is not yet in the repository. We will fix it shortly, now that the builds otherwise work.