fixes Bug 978363 - ensure rabbitmq is empty before doing any tests that touch it

I've seen this on vagrant, too.  Sometimes when an integration test ends in a failure, there are jobs left hanging around in rabbitmq.  These extra jobs are then offered to the processors in the next test run which then violates the expectations of the processor integration test.

This PR makes sure that both the normal and priority queues are empty in the *before* the integration tests are run.  The integration test also tries to clean up afterwards, but if the script ends prematurely, rabbit droppings will be left in the queues.  The pre-cleaning will take care of that mess.