Add RabbitMQ requirements, virtual environment

Before raising this MR, consider whether the following are required, and complete if so:

• [ ] Unit tests
• [ ] Metrics
• [ ] Documentation update(s)

If not required, please explain in brief why not.

Description

This adds a RabbitMQ requirements file with a dependency on pika and testing.rabbitmq.

Changes proposed in this merge request:

• Add requirements.rabbitmq.txt
• Add testing.rabbitmq to requirements.tests.txt
• tox.ini: define rabbitmq-venv virtual environment

Issues addressed

Addresses #374 (closed)