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](https://www.npmjs.com/package/dotenv-safe), with that you can define the needed env variables in the `.env.example` (the optional ones can be prefixed with `# `).