INT-3437 MQTT - Support Cluster Connection

JIRA: https://jira.spring.io/browse/INT-3437

Note: Tests are still ignored due to hung connection
on close problem.
I just let the client do the validation, it will catch an empty string. "tcp://NO_URL_PROVIDED " is needed because the client has bad logic. It validates this URI even if it is not used by the connection - the settings on the `MqttConnectOptions` override this URI but they still validate it here in the constructor :frowning:

If the user provides no URL and no serverURIs, we detect that in the adapters, so we will never try to connect to this URL.