resurrect invocation of ISupervisor.assigned() & make Supervisor.launchDaemon() accessible

As discussed in STORM-2018, that change subtly broke the storm-mesos integration framework because of the removal of the invocation of ISupervisor.assigned().

So this ticket is tracking the reinstatement of that invocation from the supervisor core code.

Also, the launchDaemon() method of the Supervisor is not public, so we had to use reflection to allow calling it from the storm-mesos integration. That should be changed too.