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/vpp/config/lcp.vpp

@@ -0,0 +1,6 @@
+comment { Create one LinuxCP Interface Pair for each phy }
+lcp create loop0 host-if loop0
+lcp create GigabitEthernet10/0/0 host-if e0
+lcp create GigabitEthernet10/0/1 host-if e1
+lcp create GigabitEthernet10/0/2 host-if e2
+lcp create GigabitEthernet10/0/3 host-if e3