Comment 4 for bug 1817775

Use case:

Model A - runs an application (openstack in this case). Part of that is a monitoring platform Nagios.
Controller model - runs controllers.

We need to monitor the state of the controllers, so we add charms to the controller model that eventually adds an nrpe subordinate, for monitoring. We link that to the Nagios application running in Model A, using a cross model relation.

We then make a change to Model A, and a clear workflow is to commit the bundle to a git repo and apply the new bundle to the running model. We can't do that at this point in time.