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.
> > +  use_proxy = @host.params['http-proxy'] ? true : false

You can use @host.param_true?("http-proxy") as well.

-- 
Later,
 Lukas #lzap Zapletal