Comment 1 for bug 2039157

So the plan is to support part (1) with the same mechanism that we have for other constraints. To allow ways to ask juju to update storage constraints (like we can `juju set-constraints` for changing mem/cpu) and have those used for any additional units (during scale up/down).

This is likely to be expressed as `juju storage-constraints size=11GB`.

This doesn't give an API to trigger the actual reallocation, but it does allow updating the "when starting a new one, make sure these attributes apply".