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.
On 12.04, it seemed to work automatically. I needed to add the explicit grub-install for 14.04 though. 

The main difference between the two was that on 12.04 we're effectively installing a new meta package, rather than just upgrading the existing one.