Support JUJU_CONTROLLER and JUJU_MODEL env vars

### Description

python lib juju doesn't support the env vars see: https://github.com/juju/python-libjuju/issues/909

Even that they don't support, COU can take those env vars and use the `[controller:][user/]model` syntax to [connect to the model](https://pythonlibjuju.readthedocs.io/en/latest/api/juju.model.html#juju.model.Model.connect). This will avoid users complaining that COU is not getting from the right model

### Charm revision

latest/stable

### Juju version

3.6

### Cloud

_No response_

### Expected behaviour

```
export JUJU_CONTROLLER=foundations-maas
export JUJU_MODEL=admin/openstack

cou plan #plan in openstack model without the need of switch

```

### Reproduce / Test

if you don't switch the controller/model cou will target the model that juju is pointing without considering the common Juju env vars

### Notes & References

_No response_

Start by tracing how COU's plan command selects the Juju controller and model, then compare that path with python-libjuju's Model.connect documentation. Implement support for JUJU_CONTROLLER and JUJU_MODEL using the controller/user/model syntax, and verify that cou plan targets the specified model without switching the active Juju model.

Written by the indexing model from the issue text.