ERROR some models are no longer visible

This looks like a race in the uploading of the credentials where it is trying to update the ones that are there. The existing default model is in the process of being removed, so is probably marked as dead, and the credential update isn't allowing for the dead model, or it is iterating over them and the model is removed from underneath it by the undertaker.

This is a great report \o/ It highlighted couple of things... Thank you, @Adam Israel!

For one, the failure itself... We have recently added a check to verify that a cloud credential works for all the models that use it whenever we update credential content. This check should work on all models. Of course, in CI environment models get destroyed and added quickly. So, the failure observed highlighted that a dead model was being picked up for this check. Thinking about it, a dead model is of no interest when checking credential - dead model's instances are already been terminated or are in the process of being terminated. The PR that excludes dead models from credential's models is https://github.com/juju/juju/pull/9284

For two, when adding a model we should not be implicitly updating cloud credential content stored on the controller from the client. This update (and/or upload) should only happen when a user explicitly states a credential via --credential option on 'juju add-model' command. When this option is not supplied, we should be using a CONTROLLER stored default credential for this user, bug # 1796618.