If the user attempts to deploy a machine which has no common address families with the host/ip in maas_url, we should refuse to deploy it, instead of happy launching it into a brick wall of failure.
The case where I hit this:
- maas_url had an IPv4 IP in it
- the IPv4 subnet had a dynamic range
- the IPv6 subnet had NO dynamic range
- (the machine had previously been commissioned when IPv6 had the dynamic range)
- The machine was deployed with its network interface configured to use the IPv6 subnet.
Deployment ran just fine, the machine rebooted, did the final stages of cloud-init, and failed horribly, because it did not have an IPv4 address as needed to talk to the (IPv4) maas_url.
There are probably other places in the code that don't take the mismatch into consideration. Refusing to deploy when there are no common address families would cover all of those cases, whatever they may be.
