MaaS on older releases need support for newer curtin images

This came up when we chatted about problems using Xenial images on Maas this morning. And it will likely become a big problem when Xenial is released. Server environments do not change that quickly, so we should expect hosts running Trusty (cloud-archive or maas from the PPA) trying to provision Xenial.

This currently is broken because the smarts of the curtin installer seems to have changed and the rootfs-tgz (the base filesystem used) no longer comes with the kernel (and modules) pre-installed. But the init phase of older curtin versions does not include steps to download a kernel from the archive. So we end up with a client that starts the final reboot without any kernel installed.