Elide passwords in grub2 plugin

Remove both plaintext and pbkdf2 passwords from grub configuration
files and command output. Since grub does not mandate any
particular location for its authentication data we have to apply
these liberaly (to all grub*.cfg as well as to all /etc/grub.d
fragments and the output of grub2-mkconfig).

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>