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.

Slightly simpler, perhaps?

if newv, oldv := cfg.defined[attr], old.defined[attr]; newv != oldv {
      if attr == "uuid" && oldv == nil {
          // Special case: we allow uuid to be set, but only if it was not already set,
          // to allow deployed environments without it to set their UUID.
          continue
     }
     return fmt.Errorf("cannot change %s from %#v to %#v", attr, oldv, newv)
}