Implement full kernel option management for Grub 2.

Our old implementation of for managing kernel options under Grub 2,
could only remove kernel options, not set them, neither to some value
or as a "bareword" option.  This limitation was because I was using
the 'regexp_replace_lines' type from the cfgfile module to try to
modify the GRUB_CMDLINE_LINUX variable in /etc/default/grub, but that
is a very complex task when using regular expressions...

Turns out there is an Augeas lens that can handle shell variables
whose value is a list of words and assignments, exactly the format
that the GRUB_CMDLINE_LINUX variable has.  So use that in the
bootloader::grub2::kernel_option definition.