Add temporary variables for multi-nic testing

* Accept ordered lists of controller and worker networkd configs
* Do not rely on these variables. They will be replaced with a
cleaner mechanism at a future date

bare-metal/container-linux/kubernetes/cl/controller.yaml.tmpl

@@ -172,6 +172,8 @@ storage:
             --net=host \
             --dns=host \
             --exec=/bootkube -- start --asset-dir=/assets "$@"
+networkd:
+  ${networkd_content}
 passwd:
   users:
     - name: core

bare-metal/container-linux/kubernetes/cl/worker.yaml.tmpl

@@ -108,6 +108,8 @@ storage:
       contents:
         inline: |
           fs.inotify.max_user_watches=16184
+networkd:
+  ${networkd_content}
 passwd:
   users:
     - name: core