juju#10363

## Description of change

2.5 accepted error forming without a formatting directive but 2.6 insists on it. Backporting to keep codebase in sync.

juju#10355

Juju 2.5 changed the default [Azure Load Balancer SKU](https://docs.microsoft.com/en-us/azure/load-balancer/load-balancer-overview#skus) from Basic to Standard. Unfortunately, models created by Juju 2.4 get stuck when their controllers are upgraded.

This change introduces a model config parameter `load-balancer-sku-name` (name chosen to be consistent with the Azure SDK naming scheme) that allows for operators to select between Basic and Standard types. It also cleans up some of the internals of the Azure provider, which should make it friendlier to newcomers to the code.

## QA steps
```
snap install juju # (if required)
snap switch juju channel=2.4/stable
/snap/juju/current/bin/juju bootstrap azure c-az
/snap/juju/current/bin/juju deploy ubuntu
```

(Assuming `juju` is this branch)

```
juju upgrade-model -m controller --build-agent
```
>```
> no prepackaged agent binaries available, using local agent binary 2.5.8.1 (built from source)
> best version:
> 2.5.8.1
> started upgrade to 2.5.8.1
>```

```
juju model-config -m controller load-balancer-sku-name=Basic
juju add-unit ubuntu
```


## Documentation changes

Required. We need to document the new model config parameter.

## Bug reference

* https://bugs.launchpad.net/juju/+bug/1824465