Bump ignition-dracut for ignition-diskful-subsequent target
    
    https://github.com/coreos/ignition-dracut/pull/151
    
    While we're here, tweak the `download-*` scripts to use `curl` instead
    of `wget` since the former is installed by default but not the latter.
    
    Also adapt spec file for not packaging any GRUB dropins anymore.