kickstart: add gPXE template, change from net0 to netX

Again to support foreman_bootdisk which needed a slight change from the default Foreman gPXE template.

The hardcoded net0 was replaced with netX, which in gPXE means the "last opened netdev".  This means when using foreman_bootdisk, its initialisation script will pick the right device, chainload to this script and continue on the right one.  When using [our documented script](http://projects.theforeman.org/projects/1/wiki/GPXE), it will open net0, chainload to this too and it'll carry on on net0.