Fixes #7733 - Remove hosts.yml fixtures and use FactoryGirl instead

Ok, this is huge, Apologies to whoever ens up having to review this. Fortunately all the changes are in test/ so we know the functionality hasn't changed. I'd like at least two reviews so that we keep confidence in the test coverage - @ares can you do the first one?

There's probably substantial improvements we can make in the Host factory traits, feel free to suggest.
My mistake, it's an Environment being added, not a Domain.