cluster/coredns: add cluster fqdn top level domain

This means that in addition to services being discoverable the 'classic'
way:

    <svcname>.<namespace>.svc.cluster.local

They are now discoverable as:

    <svcname>.<namespace>.svc.<fqdn>

For instance, on k0 you can now internally resolve:

    $ kubectl run --rm -it foo --image=nixery.dev/shell/dnsutils bash
    bash-4.4# dig +short coffee-svc.default.svc.k0.hswaw.net
    10.10.12.192

Change-Id: Ie6875b54ed6358f30f888ca0cd96e011520ace20
diff --git a/cluster/kube/cluster.jsonnet b/cluster/kube/cluster.jsonnet
index 09c3b33..a0b1aed 100644
--- a/cluster/kube/cluster.jsonnet
+++ b/cluster/kube/cluster.jsonnet
@@ -155,7 +155,14 @@
     // Calico network fabric
     calico: calico.Environment {},
     // CoreDNS for this cluster.
-    dns: coredns.Environment {},
+    dns: coredns.Environment {
+        cfg+: {
+            cluster_domains: [
+                "cluster.local",
+                fqdn,
+            ],
+        },
+    },
     // Metrics Server
     metrics: metrics.Environment {},
     // Metal Load Balancer