20-grub-install: Always use fedora as the boot entry --class
    
    This is only used by themes and these assume that boot entries for Fedora
    will be named "fedora". Currently we are using "kernel" that's not useful.
    
    Resolves: rhbz#1957014
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>