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/hosts.j2

@@ -0,0 +1,7 @@
+127.0.0.1       localhost
+127.0.1.1       {{node.hostname}}.lab.ipng.ch  {{node.hostname}}
+
+# The following lines are desirable for IPv6 capable hosts
+::1     localhost ip6-localhost ip6-loopback
+ff02::1 ip6-allnodes
+ff02::2 ip6-allrouters