Option to give shorter image/version names for flintlock spec

[Callisto13]

Right now users have to give the full URL of images to the flintlock spec, which is annoying.

ghcr.io/weaveworks/flintlock-kernel:5.10.77

At the CAPMVM level users should be able to do something like this:

kernelVersion: 5.10.77
kubernetesVersion: 1.21.8

and we should build the rest of the url and supply flintlock with the correct things.

The previous API keys will still be available, for backwards compatibility and in case users want to bring their own images.