Split the environment concept into environment and layout

Initially, we had many environments with different nodes, etc. Nowadays,
they're all the same, except for network, image and flavour ids.

I've put the node stuff into environment/full.yaml and put the
environment specific stuff into a map file.

This depends on the corresponding change to python-jiocloud getting
merged first, though.
That's a good point. I just realised that we never ever used that feature, because we don't have an "acceptance" environment. We have an "at" environment, so we're clearly never using the fallback.

I'll make it fail more gracefully. Good call.