Add non-overlapping option to % rollout with Feature Flags

Problem to solve

Running multiple experiments at the same time is hard. We should avoid overlapping A/B/n tests and ensure our tests are exclusive. Right now the salt is based on group_id + user_id but that would be great to organize them to namespaces.

Proposal

As a second iteration to % rollout with Feature Flags, introduce namespaces to be able to run many experiments at the same time.

More about how namespaces should work: https://facebook.github.io/planout/docs/namespaces.html

What does success look like, and how can we measure that?

Links / references