Add support for LIVEBOOK_DISTRIBUTION=none

[Munksgaard]

It seems like mix releases have support for RELEASE_DISTRIBUTION=none, which means that distribution is disabled. I am wondering if there is a particular reason why livebook does not support that flag, or at least why the documentation does not mention it.

I would also expect, if LIVEBOOK_DISTRIBUTION is set to none, that the releases/COOKIE file is not written to.

[josevalim]

Livebook uses the Erlang distribution to talk to each runtime inside the notebook, so that's simply not possible.

[Munksgaard]

Aha, thank you for clarifying.