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.
