Utilize sd-nd-wait-online for waiting on DNS servers

[slyon]

Description

networkd: wait-online wait for DNS servers to be assigned

As implemented in systemd (v258):
systemd/systemd#34640

Spec: https://discourse.ubuntu.com/t/spec-definition-of-an-online-system/27838
Follow-up to: #482 #456

Checklist

• Runs make check successfully.
• Retains code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

@enr0n and @daniloegea would you mind taking a look at this PR? I'll still keep it as a draft (please don't merge yet), as we need wider availability of the upstream systemd changes, especially in our test runners (i.e. systemd v258+ or backports).

But besides that, the PR should be ready for review and comply with the spec linked above.

Danilo, do you have any clue what's wrong with the CodeQL (cpp) workflow? Just upgrading from v2 to v3 doesn't seem to do the trick.

[slyon]

The systemd part of this work was landed upstream and backported to https://launchpad.net/ubuntu/+source/systemd/257.2-3ubuntu1, thanks to @enr0n !

I've now finished implementation of the Netplan side and tested it in Ubuntu Plucky (by uncommenting the line after # XXX: Enable extra testing once systemd#34640 is available on the SUT (i.e. systemd v258+). in ethernets.py autopkgtest), so this should be ready for review!

Before merging it, we need to make sure the CI is ready (i.e. systemd v258+ or backport of the feature) and we should also run the full autopkgtest, as described above.

@enr0n WDYT?

[slyon]

This is now included in Ubuntu Plucky: https://launchpad.net/ubuntu/+source/netplan.io/1.1.2-1ubuntu1