[BB-3729] Bug: OCIM allots a RabbitMQServer randomly and ignores the accepts_new_clients flag

This adds tests that verify new OpenEdXInstance instances are correctly assigned RabbitMQ instances that have accepts_new_clients=True.

Related issues

• BB-3729

Testing

make test.one make test.one instance.tests.models.test_openedx_instance.OpenEDXInstanceRabbitMQTestCase