9d06bb890f
Currently interacting with functions in zaza.model requires the model to be passed in. This has resulted in multiple calls to get_juju_model(). It is cleaner to push these calls down into the model wrapper and make the model_name an optional argument. In addition, the current model name is now cached without having to check the os.env each time. Unfortunately this has resulted in the signature changing on a great many function so this diff is bigger than would normally be desirable.