Adds messagebroker-config to make file

Fixes #3102 

As a part of the Drupal Message Broker refactoring:

https://jira.dosomething.org/browse/MB-45
https://github.com/DoSomething/message_broker_producer/issues/44

- Access to `mb_config.json` from the `messagebroker-config` repo is required for the `message_broker_producer.module`.