Refactor enumeration of deployed releases

As mentioned in #994, our project structure requires folders other
than actual releases to exist in the releases_path. To be able to
customize the logic to enumerate releases (to exclude said non-
release folders), it has been refactored into a "configuration lambda"
`:releases`.

:warning: We haven't extensively tested this change yet. I hope that I didn't introduce any subtle errors, but you never know :smile: Hopefully some people can test this with their deployment to see if it breaks anything. The affected tasks should be `deploy:cleanup` (and thus implicitly `deploy`) and `deploy:rollback`.

I would write unit tests, but there doesn't seem to be a SSHKit backend suitable for that purpose at the moment. If there is and I just have missed it, please point me to it!