livecd.py: Add systemd modules to dracut
livecd.py: Add spacing before 'menu default'

Make isolinux.cfg look better.
livecd.py: Prepare more isolinux options

But don't use them just yet.
livecd.py: Add default kernel arguments for dracut
livecd.py: Remove rhgb and add splash command line argument
livecd.py: Add 'splash' kernel command line argument for plymouth-lite