Move DHCP service to dns1.p.b

The DHCP server on the UniFi Security Gateway is pretty limited; it
cannot manage static leases (reservations), and does not offer any way
to build dynamic values for e.g. hostname or boot filename. Rather than
give up these features, I decided to just move the DHCP server to one of
the Raspberry Pis; the DNS server made the most sense.

To facilitate this move, I created the *pyrocufflink-dhcp* host group,
and moved the DHCP configuration variables there. Thus, it was a simple
matter of adding *dns1.pyrocufflink.blue* to this group to relocate the
service.

Of course, to serve clients on the other subnets, the gateway needs to
have DHCP relay enabled and pointing to the new server.

hosts

@@ -28,7 +28,7 @@ dch-gw
 vm-hosts
 
 [dhcpd:children]
-dch-gw
+pyrocufflink-dhcp
 
 [file-servers]
 file0.pyrocufflink.blue
@@ -82,6 +82,9 @@ vmhost0.pyrocufflink.blue
 web0.pyrocufflink.blue
 zbx0.pyrocufflink.blue
 
+[pyrocufflink-dhcp]
+dns1.pyrocufflink.blue
+
 [pyrocufflink-dns]
 dns0.pyrocufflink.blue
 dns1.pyrocufflink.blue