fix: possible AMQPUser and compute config mismatch

If we rolled out a new yaook release that changed the RabbitMQ username of the compute AMQPUser, mismatches between the AMQPUser RabbitMQ username and the RabbitMQ username inside the nova-compute config could occur. This only happened when the AMQPUser was rolled out by the previous Yaook release and the config secret was created with the new release. To ensure this does not happen, we now use the k8s API to retrieve the currently used RabbitMQ username to create the compute config.

Instead of using a separate TransportLayers for NovaComputeNodes, we could always retrieve the username using the k8s API (not sure if we want that though).

Closes #653 (closed)