So yeah, I had just documented in the README.md for rspec-system: https://github.com/puppetlabs/rspec-system#vagrant-provider

Slightly off-topic: So one of the things I have often thought about is the value of using a prefix for environment variables. This gives us the ability in the future to load all environment variables using a particular prefix, and perhaps validate them. Also, like with options (ie. the --type discussion) there could be value in having 'plugins' pre-declare configuration items also *shrug*. I didn't go much beyond these thoughts though, what you see here for rspec-system is just a convention and has no internal meaning. Anyway its an interesting idea :-).

Regardless, hen pecking for options from ENV seems to reek of 'global-unstructured-evilness' to me but I'm not sure why, but I get the impression we'll get bitten by it eventually. I guess however its not a problem today.