HTTP proxy for repositories (kickstart/preseed)

This introduces two new variables:
- http-proxy
- http-proxy-port

This proxy is then used to fetch stuff from the mirror. It's intentional that the communication to the Foreman instance is not using the proxy.

This is thought more as a WIP for discussion.
I don't think so, as http-proxy is a string with the fqdn of the proxy, not true/false.