When we convert the OpenStack cloud provider to run in an external process, we can now use the kubernetes Secrets capability to inject the OS_* variables. This way we can specify the cloud configuration as a configmap, and specify secrets for the userid/password information. The configmap is mounted as a file, and the secrets are made available as environment variables. The external controller itself runs as a pod/daemonset. For backward compatibility, we preload all the OS_* variables, and if anything...

* Allow path parameters to have default values (e.g. `None`) and discard them instead of raising an error.
    * This allows declaring a parameter like `user_id: str = None` that can be taken from a query parameter, but the same *path operation* can be included in a router with a path `/users/{user_id}`, in which case will be taken from the path and will be required.