Fix fb_systemd on hosts without networkd (#236)

Test Plan:
Tested that this didn't result in a networkd config change on CentOS9, and
that it stops managing the networkd config on CentOS8, which is fine since we
aren't using networkd on CentOS8 at least internally.

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

Differential Revision: D55154300

fbshipit-source-id: db4822e2df0ab65258a204c881afd042898cd0a5

cookbooks/fb_systemd/recipes/networkd.rb

@@ -19,6 +19,7 @@
 #
 
 fb_helpers_gated_template '/etc/systemd/networkd.conf' do
+  only_if { node['fb_systemd']['networkd']['enable'] }
   allow_changes node.nw_changes_allowed?
   source 'systemd.conf.erb'
   owner node.root_user