Make `bind` default value for all other `bind` options

[spuun]

Today there is the ini option bind in the [main] section (also available as cli -b / --bind) which will set amqp_bind and http_bind. However, there are more bind options (mqtt_bind, metrics_http_bind, clustering_bind) which one must set explicit.

I suggest that bind act as a default value for all bind options.

This will probably need some refactoring making all bind options nilable with nil as default value, then add an explicit getter which fallbacks to bind.