GH-151: Remove output bean from the Rabbit Supplier

Fixes https://github.com/spring-cloud/stream-applications/issues/151

The `RabbitSupplierConfiguration` comes with the `output` bean definition which
clashes with the `output` binding for Spring Cloud Stream.
The last one doesn't register the binding bean and leave the rest of the logic
to deal with existing bean when we resolve it by the `output` name.

* Rework the logic of the `RabbitSupplierConfiguration` to avoid an extra bean
in between and the binding in Spring Cloud Stream do it job

applications/source/rabbit-source/src/test/java/org/springframework/cloud/stream/app/source/rabbit/RabbitSourceListenerTests.java

@@ -77,6 +77,7 @@ public class RabbitSourceListenerTests {
 						"--spring.rabbitmq.listener.simple.acknowledgeMode=AUTO",
 						"--spring.rabbitmq.listener.simple.prefetch=10",
 						"--spring.rabbitmq.listener.simple.transactionSize=5",
+						"--spring.cloud.stream.function.bindings.rabbitSupplier-out-0=output",
 						"--spring.rabbitmq.port=" +
 								"${spring.rabbitmq.test.port}"
 				)) {