Merge pull request juju#16105 from SimonRichardson/inject-ctrl-config…

…-upgrade-steps

juju#16105

The following injects the controller config getter into the upgrade steps facade. This is a nice easy example of swapping over to the controller config service.

## Checklist

- [x] Code style: imports ordered, good names, simple structure, etc
- [x] Comments saying why design decisions were made
- [x] Go unit tests, with comments saying what you're testing

## QA steps

```sh
$ juju bootstrap lxd test --build-agent
$ juju add-model ubuntu
$ juju deploy ubuntu -n 3
$ juju upgrade-controller --build-agent
$ juju upgrade-model
```