A networking setup snippet is required for preseed OSes that can configure interfaces either inside the main preseed configuration, or be called from a finish script.
Here's the kickstart version for reference: https://github.com/theforeman/community-templates/blob/master/snippets/kickstart_networking_setup.erb
Alternatively, we could combine them into something more cross-platform such as netcf (http://projects.theforeman.org/issues/7460).
I put something together that seemed to work (in ~1.7). There was the issue with debian installer that removed customized interfaces file just before reboot, so we didn't merge it. If there's someone who'd like to work on this, here's the gist. Also here's the potential workaround for the reboot issue http://projects.theforeman.org/projects/foreman/wiki/Tips_&_Tricks#Debian-based-Tips
add preseed_networking_setup snippet