add optional grublocal.template for disabling netboot in efi/grub setups

I have cobbler working with a bunch of UEFI boxes except that I could not disable netboot. This patch fixes that by letting me write a grublocal.template file that is used to generate confs in /var/lib/tftboot/grub when netboot is disabled. 

I wanted to add a grublocal.template as well, but I am having difficulties writing a generic one that works like pxelocal.template. For the time being, I'm just making grublocal.template optional.