enabling obersvability plugin after scaling out does not give new juju users access to cos model

[Nishant Dash]

If I have a, say, a single node sunbeam cluster and then scale out by one node for example, I can see that a new juju user is successfully created for the new node and it has access to both controller and openstack models.

However at this point, if I enable the observability plugin (which adds a new model and deploys COS), I would expect sunbeam to take care of updating permissions for the new node's juju user to also see this model but as of my current testing I need to manually run the `juju grant` command against the new node juju user to allow it access to the observability model.

However, even without that I can still see the obs model with read level access, so if that is the intended effect, then the only missing part I feel is the fact that `.local/share/juju/models.yaml` for example, does not have infromation about the observability model.