Set dnsPolicy to Default on k8s build jobs

This prevents the builder pods from having resolv.conf pointed at the
kube-dns service, which they won't have access to.

buildman/manager/executor.py

@@ -384,6 +384,7 @@ class KubernetesExecutor(BuilderExecutor):
             ],
             'imagePullSecrets': [{'name': 'builder'}],
             'restartPolicy': 'Never',
+            'dnsPolicy': 'Default',
           },
         },
       },