Fix docker MTU configuration when no MTU specified

We'll omit the option and use Docker's default value.

ansible/docker.yml

@@ -3,4 +3,4 @@
   hosts: docker
   roles:
     - role: docker
-      docker_daemon_mtu: "{{ external_net_name | net_mtu }}"
+      docker_daemon_mtu: "{{ external_net_name | net_mtu | default }}"

ansible/roles/docker/templates/daemon.json.j2

@@ -1,6 +1,8 @@
 {
   "storage-driver": "devicemapper",
+{% if docker_daemon_mtu %}
   "mtu": {{ docker_daemon_mtu }},
+{% endif %}
   "storage-opts": [
     "dm.thinpooldev=/dev/mapper/{{ docker_storage_volume_group | replace('-', '--') }}-{{ docker_storage_volume_thinpool | replace('-', '--') }}",
     "dm.use_deferred_removal=true",