Comment 5 for bug 1664611

The fix for this is a little bit complicated, since there's interaction between $JUJU_MODEL, the current-controller value in the controllers.yaml file, and the -c flag.

$JUJU_MODEL may or may not specify a controller, i.e. it can have the syntax "controller:model", or just "model". In the latter case, the current controller is implied.

I think what we should do is prohibit use of "-c" if $JUJU_MODEL is specified, and set the controller name to the controller specified or implied by $JUJU_MODEL.