ceph-mon charm runs into error when using focal-ussuri

Reproducer:
```
cd stsstack-bundles/ceph
./generate-bundle.sh -n ceph-octopus --no-openstack --run
```

And `ceph-mon` will end up in error state. Log says:
```
charmhelpers.fetch.sourceconfigerror: unsupported cloud: source option focal-ussuri
```

Changing the `source` option to `distro` "fixes" it.