Adds message_broker_producer_load_config_options()

Fixes #44 

Second attempt...

- Adds hook_libraries_info entry for `messagebroker-config`
- Adds `message_broker_producer_load_config_options()` to load `mb_congif.json` settings
- Stubs out call to `..._load_config_options()` in `message_broker_producer_config_form()`