Allow to set all cubes as private by default

[igorlukanin]

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.