dch-gw: Clean up DNS/NTP address enumerations

Instead of listing the addresses for DNS and NTP servers again in the
DHCP server configuration, these are now taken from the canonical
definitions in the `dch_networks` variable.

group_vars/dch-gw/dhcpd.yml

@@ -8,14 +8,10 @@ dhcp_subnets:
     end: 172.31.0.30
   routers:
   - 172.31.0.1
-  dns_servers:
-  - 172.31.0.4
-  - 172.31.0.10
+  dns_servers: '{{ dch_networks.jazz.dns_servers_v4 }}'
   domain_name: pyrocufflink.jazz
   domain_search: pyrocufflink.jazz
-  ntp_servers:
-  - 172.31.0.4
-  - 172.31.0.10
+  ntp_servers: '{{ dch_networks.jazz.ntp_servers }}'
 
 # pyrocufflink.blue
 - address: 172.30.0.0/26
@@ -24,12 +20,10 @@ dhcp_subnets:
     end: 172.30.0.59
   routers:
   - 172.30.0.1
-  dns_servers:
-  - 172.30.0.4
+  dns_servers: '{{ dch_networks.blue.dns_servers_v4 }}'
   domain_name: pyrocufflink.blue
   domain_search: pyrocufflink.blue
-  ntp_servers:
-  - 172.30.0.10
+  ntp_servers: '{{ dch_networks.blue.ntp_servers }}'
 
 # pyrocufflink.red
 - address: 172.31.1.0/24
@@ -38,14 +32,10 @@ dhcp_subnets:
     end: 172.31.1.254
   routers:
   - 172.31.1.1
-  dns_servers:
-  - 172.31.0.4
-  - 172.31.0.10
+  dns_servers: '{{ dch_networks.red.dns_servers_v4 }}'
   domain_name: pyrocufflink.red
   domain_search: pyrocufflink.red pyrocufflink.blue pyrocufflink.jazz
-  ntp_servers:
-  - 172.31.0.4
-  - 172.31.0.10
+  ntp_servers: '{{ dch_networks.red.ntp_servers }}'
   dynamic_hostnames: true
   default_lease: 3600
   max_lease: 28800