Get iPXE script template kernel parameters from ISO

Currently, we have the kernel parameters in the iPXE script statically defined from what Assisted Service generates. If the default parameters were to change in RHCOS that would be problematic. Thus, it would be much better if we were to extract them from the ISO.

The kernel parameters in the ISO are defined in EFI/redhat/grub.cfg (UEFI) and /isolinux/isolinux.cfg (legacy boot)