As mentioned by @hatch, there is currently no command to deal with this.
You can however do it manually.
All of the lxc containers are named after the user and the name of the environment.
So let's say that your user-id is 'tim' (mine is), and the environment is 'testlocal', if you have three machines (other than machine 0 which is actually the host), you can go:
sudo lxc-stop -n tim-testlocal-machine-1
sudo lxc-stop -n tim-testlocal-machine-2
sudo lxc-stop -n tim-testlocal-machine-3
You can then stop the machine agent and db server on the host machine.
sudo stop juju-agent-tim-testlocal
sudo stop juju-db-tim-testlocal
To bring it back up, just reverse the steps:
sudo start juju-db-tim-testlocal
sudo start juju-agent-tim-testlocal
And then bring up the lxc containers
sudo lxc-start -n tim-testlocal-machine-1
sudo lxc-start -n tim-testlocal-machine-2
sudo lxc-start -n tim-testlocal-machine-3
That should be sufficient.