Definition for managing Grub 0.x options.

This adds the defintion bootloader::grub0::option, which can set and
unset options in the Grub 0.x config file.  It handles both "bare"
options (e.g. 'hiddenmenu') and valued options (e.g. timeout=5).

Managing Grub 2 options seems a bit more complicated, so for now we
only implement the Grub 0.x version.