dotenv-safe instead of manual checking a hardcodet whitelist

Instead of: https://github.com/wearehive/project-guidelines/blob/4baf60bab3fc3d4616712b04f3dab48a3774d20c/config.sample.js#L1-L9

Maybe its an idea to use dotenv-safe, with that you can define the needed env variables in the .env.example (the optional ones can be prefixed with # ).