Fix fb_systemd on hosts without networkd

[jaymzh]

Fix fb_systemd on hosts without networkd

Summary:

If you don't use networkd (and fb_networkd), then this code in fb_system::networkd

Test Plan:

@anitazha did some testing of this internal to Meta already, plus I've rolled this out to the SCALE infra

[facebook-github-bot]

@joshuamiller01 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.