* Add calls to asyncio.get_event_loop().close() to close loop. libjuju does
not do this for you and needs to be done just before exiting.
* Require model to be explicitly set when running command line tools. This is
part of the drive to ensure that eventually multiple runs can be performed
concurrently.
* Add set_juju_model/get_juju_model functions to manage which model is in
focus