Protect rabbit transport definition
Protect rabbit transport definition

As it turns out, a plugin can be loaded several time during a CI run.
When it does so, the trick used to save the rabbit get_transport_url
method definition erases the previous version. This protects it by only
defining it once.

Change-Id: I7eb5a1934b4be87b289df8c78927852fc84aac18

openstack/devstack-plugin-amqp1