fix: cloud entries in an dictionary were being deleted while iterating the object

[addyess]

Found in a PR where multiple kubernetes clouds were created by pytest_operator atop an existing machine cloud.

When cleaning up the created clouds, we deleted an entry in self.forget_cloud(...) that was maintained by self._clouds

this results in an exception:

  File "/home/ubuntu/actions-runner/_work/k8s-operator/k8s-operator/.tox/integration/lib/python3.12/site-packages/pytest_operator/plugin.py", line 1092, in _cleanup_models
    for cloud in reversed(self._clouds):
RuntimeError: OrderedDict mutated during iteration

The correct behavior is to operate on an inline list instead of iterating the dict itself.