environs/config: introduce config.UUID()

Introduce `config.UUID()` method to access the uuid of the environment.
    
A uuid is the canonical identifier for an environment, and superceeds the environment's name.
    
For backwards comparability, `config.UUID()` is currently permitted to return "", but once all environments are converted to have an environment uuid in place, this method will change to return a valid uuid for a valid config.

I think we agreed (string, bool)