Scaleway: Force on-link: true for static networks (#5523)

[karibou]

On some distributions like focal (See LP: #2073869) systemd fails to add a route which is not in the subnet of the NIC if onlink is not explicitely specified.

Force on-link to true to avoid those situations.
Fixes: GH-5523

Proposed Commit Message

Scaleway: Force on-link: true for static networks (#5523)

<type>(optional scope): <summary>  Force use of on-link: true not working on Ubuntu Focal

The version of systemd on Ubuntu Focal does not support subnets outside
of the NIC network without addition of on-link. This is not the case with other Ubuntu
distributions.

Fixes GH-5523
LP: #2073869

## Merge type

- [x] Squash merge using "Proposed Commit Message"
- [ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[TheRealFalcon]

@karibou , thanks for this PR. The changes look good to me, though it conflicts with a PR that was just landed. Would you be able to rebase?

[karibou]

Hello @TheRealFalcon ,

Sorry for the delay, I didn't see your update.

Rebase is done with a fixed unittest.

[TheRealFalcon]

LGTM!