Support for caas namespaces

[wallyworld]

Description of change

Each CAAS model's k8s resources (pods, services, etc) are now created in a namespace named after the model. Because we don't have a model manager worker yet to react to model add/remove, we create and delete the namespace inside the k8s client when an operator is created/deleted. This is a workaround until the necessary model worker can be written.

To provide the model name to the k8s broker tracker, we implement an existing TODO which is to add a bespoke caas agent facade. There's also a driveby fix to clean up the caas operator provisioner worker logic a bit.

QA steps

Run up a CAAS model and check that gitlab/mysql can be deployed and related, and that the expected namespace is used. Delete all model applications and check that the namespace is removed.

[babbageclunk]

This looks ok.
(It seems crazy to me that there isn't already some way for a worker to get this information - or that it takes so many lines of code + tests to expose the model name to a worker.)

[wallyworld]

$$merge$$

[wallyworld]

$$merge$$