So far the listener stack and the pool stack were under the same
class (Listener), which caused an unnecessary coupling between
them as these are separate entities.
This patch separates between the listener stack and the pool
stack by moving the pool part to a new file:
tobiko/openstack/stacks/octavia/pool.yaml
Change-Id: Ia5e3cca6b543e5907cc875d2e7dbb4f7424d9c9e