Framework for linux kernel upgrade

... so that we can fix security issues and move to better, more recent,
kernels without reinstall.

NB: There is a problem with most of our VM templates that means we're on
grub-legacy (v1), which in turn was preventing the newer kernels from being
selectable on boot. Hence, make upgrading grub to grub-pc (v2) a prereq.
Do we need to run grub-install after every kernel upgrade? Doesn't the package install normally handle this for us?