Provide a clean way to get a reference to the MessageBus running in SingleNodeApplication

Currently the message bus is only obtained via Module.getComponent(MessageBus.class). Stream testing scenarios that depend on sending and receiving payloads via named channels do not require a deployed module instance per se, but any stream flow control uses the MessageBus directly. Getting a deployed module instance in general is expensive, e.g., you have to wait for the module to deploy asynchronously, whereas the MessageBus implementation could be known a priori when the application starts. An improvement would be to ask the container for its MessageBus.