Remove unused kernels

Based on the output of govuk_unused_kernels, allow Fabric to support the
removal of unused kernels which often take up a lot of disc space.
I don't blame you, because vm.py is full of examples of `run('sudo blah')` but you might like to give it a try with `sudo('govuk_unused_kernels | xargs apt-get purge -y')` as the fabric sudo handler has a lot of advantages (for a start you can pass it options via flags and environment variables).