juju upgrade-juju fails if agents are not up to date

If you ever have a model where some agents are stopped/not responding, it can become impossible to upgrade the rest of the model. This also happens if you have machines that are flagged as pending that never actually get provisioned.

The source of this is that we check the agent versions of all unit and machine agents and only upgrade if all of their versions match exactly our current model version.

We should provide some way to upgrade models in these states, as often it requires an upgrade in order to fix the problems.