MAAS metadata_url contains a domain name even when DNS resolution is disallowed

[Ghadi Rahme]

[MAAS Version]
Tested on MAAS snap 3.4/edge (v:3.4.0-14305-g.3c4f0ec35 R:30809)
Ubuntu 22.04

[Description]
Installing MAAS 3.4/edge and enabling DHCP will cause machines to fail commissioning with the following error:

cloudinit.sources.DataSourceNotFoundException() Did not find any data source

After further investigation, it turns out that the metadata URL being shared by MAAS to the machines contained a domain name instead of the MAAS server IP address.

Expected URL (for my setup):
http://192.168.122.243:5248/MAAS/metadata/

Actual (bad) URL:
http://192-168-122-0--24.maas-internal:5248/MAAS/metadata/

The option "Allow DNS resolution" in MAAS is disabled and no DNS servers are specified in MAAS. This means that even when using an external DNS server MAAS will keep sending the URL with the domain name which will fail to resolve and cause the commissioning to fail.

All testing was done in libvirt/KVM VMs

[Work Around]
I was able to work around the issue by adding a DNS entry to my home DNS pointing to the MAAS server and modifying the libvirt network to use that DNS instead.

[Jacopo Rota (r00ta)]

we use domain name if
```
def _subnet_uses_dns(subnet):
    return (
        subnet is not None
        and not subnet.dns_servers
        and (subnet.vlan.dhcp_on or subnet.vlan.relay_vlan_id is not None)
    )
```

and this is why in your case we don't return the rack IP directly. We should add a check on the `allow_dns`. I'll fix this

[Jacopo Rota (r00ta)]

Mattermost conversation here https://chat.canonical.com/canonical/pl/7c16zutpabn1fk7y9ycrquh79y