Allow to set all cubes as private by default

**Is your feature request related to a problem? Please describe.**
From a conversation on slack.cube.dev:

> According to the best practice we would like to set `public: false` to all cubes and only expose views. Do you know if there's a way to set all cubes `public: false` globally so we don't need to define them one by one?

**Describe the solution you'd like**
It would be great to have an environment variable to enforce that behavior globally across the deployment.