Remove test around non-existing exchange.

The desired behaviour (content-store does not create the exchange) is
now handled by rabbitmq permissions - the content_store is not allowed
to create exchanges
Add per-Rails.env sections to the rabbitmq config.

Also update the integration tests to use server-generated non-durable
queues.  This ensures that the tests are isolated from each other, and
that the queues are deleted after each test run.
Add task to watch the message queue messages