Confine GRUB providers to presence of menus, prefer GRUB 2

On a GRUB 2 system, the GRUB 1 provider will always match as it's only confined
on the presence of Augeas and so the behaviour is often random, it might choose
the older provider:

    Warning: Found multiple default providers for kernel_parameter: grub,
    grub2; using grub

Since we'll never create GRUB configurations from scratch, it makes sense to
confine these providers to their files, and prefer GRUB 2 over GRUB 1.