netplan does not allow a top-level 'nameservers' stanza

[Steve Langasek]

The netplan draft spec calls for 'nameservers' to be allowed as a top-level stanza. It is currently only implemented as a per-device configuration.

While we've discussed that the nameserver addresses should really just be associated with the interfaces which route to those addresses, 'nameservers' is also how you define the DNS search path as would be included in /etc/resolv.conf. A machine's DNS search path is not a function of which interfaces are currently up, and needs to be globally definable.

If we decide this is client configuration that's out of scope for netplan, we should clarify the spec to not list 'nameservers' at the top level.

If we decide that it is in scope, but that we still don't want to support declaration of global nameservers just the search path, we should fix this to be e.g. 'domain-search' at the top level.