fix up the nat server role to operate correctly in check mode

Former-commit-id: 6123b104

roles/nat_server/tasks/main.yml

@@ -23,6 +23,8 @@
   # output looks like
   # 8.8.8.8 via 118.138.254.254 dev eth2  src 118.138.254.185
   shell: /usr/sbin/ip route get 8.8.8.8  | awk '{print $5;exit }'
+  check_mode: no
+  changed_when: false
   register: public_device_name
 
 #if not defined, default to M3=vlan 114  ;