Live migrations fail on CIS hardened environments

On CIS hardened environments, root login is disabled in /etc/ssh/sshd_config with:

PermitRootLogin no

that makes live migrations failing regardless VMs are on shared storage or not.

The error is:

```
2023-10-17 08:34:42.602 3904 ERROR nova.virt.libvirt.driver [-] [instance: 67ba8b07-22b7-4d4b-bf47-2a15d3f4bc45] Live Migration failure: Cannot recv data: Authorized uses only. All activity may be monitored and reported.
root@10.10.10.23: Permission denied (publickey).: Connection reset by peer: libvirt.libvirtError: Cannot recv data: Authorized uses only. All activity may be monitored and reported.
2023-10-17 08:34:42.815 3904 WARNING nova.compute.manager [req-bf622559-f199-4a2d-948f-3962bf686c4d 56b7446f04fa446ca90b1599c729bbe6 c86bb3408a9743f3989b76007787e089 - 4addaf4e7f7d4915bf2387f12bd2b42b 4addaf4e7f7d4915bf2387f12bd2b42b] [instance: 67ba8b07-22b7-4d4b-bf47-2a15d3f4bc45] Received unexpected event network-vif-plugged-b2d4198a-54c2-478d-aa85-39453475a78e for instance with vm_state active and task_state migrating.
2023-10-17 08:34:43.014 3904 ERROR nova.virt.libvirt.driver [-] [instance: 67ba8b07-22b7-4d4b-bf47-2a15d3f4bc45] Migration operation has aborted
2023-10-17 08:34:43.034 3904 ERROR nova.compute.manager [-] [instance: 67ba8b07-22b7-4d4b-bf47-2a15d3f4bc45] Did not find resource allocations for migration 5413f94a-aaac-4359-82b5-81976c0d1c0d on source node compute-9.domain. Unable to revert source node allocations back to the instance.
```

Charm should override that settings adding a config to /etc/ssh/sshd_config.d

with something like

```
PermitRootLogin yes
Allowusers root@<libvirt-migration-network-ip>
```