Futher flesh out the generator

- Create a per-host directory called overlays/$(overlay)/hostname/$(host.hostname) to have files that
  ought to be included only for that host. Things like /etc/vpp/config/interface.vpp go there
- Rename the "templates" directory as overlays/$(overlay)/common/
- Render one after the other, so a file can exist in common and hostname, the latter taking precedence
- Remove the config for 'pubkeys' and instead just make these common/root/.ssh/* and common/home/ipng/.ssh/*
- Split out bootstrap.vpp so that a per-host include can be overriden for interfaces.vpp
  - But keep a default common/etc/vpp/config/interface.vpp as a placeholder, so that VPP will still start
    even if the per-hostname override isn't provided.
- Generate a fresh output for 'default' on all machines

overlays/bird/common/etc/netplan/01-netcfg.yaml.j2

@@ -0,0 +1,14 @@
+network:
+  version: 2
+  renderer: networkd
+  ethernets:
+    enp1s0:
+      optional: true
+      accept-ra: false
+      dhcp4: false
+      addresses: [ {{node.mgmt.ipv4}}, {{node.mgmt.ipv6}} ]
+      gateway4: {{lab.mgmt.gw4}}
+      gateway6: {{lab.mgmt.gw6}}
+      nameservers:
+        addresses: [ "2001:678:d78::3", "2001:678:d78::4" ]
+        search: [ "lab.ipng.ch", "ipng.ch", "ipng.nl", "rfc1918.ipng.nl" ]