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.
TBH i found that factory online and it tested fine for me so I used it. Happy to make adjustments if you think it's better ;)