sunbeam command doesn't warn about missing FQDN

[Nobuto Murata]

Looks like there is an assumption that each node must have FQDN.

e.g. https://microstack.run/docs/single-node
> Note: If your host is a virtual one managed with Multipass, you will need to ensure it has an FQDN set in the /etc/hosts file. See bug GH #3277.

I think having FQDN everywhere is a good thing, but the problem here is sunbeam command doesn't stop nor warn about it even when a node is missing FQDN. It would be good to check those in a pre-flight stage so users can clearly recognize the necessity of setting FQDN and they can fix it at the earliest stage before going further.