Add support to kubernetes-nmstate for deleting the changes made by an NNCP when that NNCP is deleted.

Why is this important?

Users find it confusing that deleting an NNCP does not remove the changes it made. This breaks the GitOps workflow, as one example.

Basically every person I talked to about kubernetes-nmstate at the f2f mentioned this. While we do document this behavior, it would be far better if the operator just worked the way users expect.