code: def resolve(self, config_key):
          
          for config_source in self._config_sources:
              value = config_source.resolve(config_key)
              if value:
                  return value
  
          return None
docs: Resolve the value of the given config parameter key. Key must be correctly scoped for
          Lexicon, and optionally for the DNS provider for which the parameter is consumed.
          For instance:
              * config.resolve('lexicon:delegated') will get the delegated parameter for Lexicon
              * config.resolve('lexicon:cloudflare:auth_token') will get the auth_token parameter
                consumed by cloudflare DNS provider.
  
          Value is resolved against each configured source, and value from the highest priority source
          is returned. None will be returned if the given config parameter key could not be resolved
          from any source.