Convergence feature flag evaluation function

Just a function to make the determination; not hooked up to anything.
Right now there is no other configuration in convergence though.  And I sort of hope we don't add any - configuration is only a good idea if there are multiple ways in which the software can be deployed, and the appeal of convergence is that it should behave consistently.  Although perhaps this points in the opposite direction - maybe the schema should be something like this instead:
```json
{"feature_flags": {"a_tenant_id": ["convergence"], "another_tenant_id": ["convergence", "rolling_updates"]}}
```
Thoughts?