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 see nowhere that you check `uri` for empty, just for `null`.
Does that comment above mean that we rely on that `Client validation`?
What is a purpose of this `tcp://NO_URL_PROVIDED` ? To fail validation ?