When bootstrapping fails, jenv files are now cleaned up.

https://bugs.launchpad.net/juju-core/+bug/1361680
Good point. After speaking with @natefinch, I think the pattern we'll go with is: environFromName for the var, and environFromNameProductionFunc for the actual function. It's ugly to dissuade use directly, and it's somewhat descriptive.

For the record, I'm not overly fond of this way of mocking. It couples us tightly with our implementation, and it introduces strangeness into the production code.