ktest: Add support for grub2

As only grub or 'script' is supported for rebooting to a new kernel,
and Fedora 17 has dropped support for grub, I decided to add grub2
support as well (I also plan on adding syslinux/extlinux support too).

The options GRUB_FILE and GRUB_REBOOT were added to allow the user
to specify where to find the grub.cfg and what tool to use to reboot
into the next kernel respectively.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>