Only set subnet if explicitly defined

defaults/main.yml

 docker_pip_packages:
   - 'docker'
   - 'docker-py'
-docker_subnet: '172.17.0.0/24'
+docker_subnet: false
 docker_compose_version: '1.17.1'

tasks/install.yml

@@ -36,7 +36,7 @@
 
 - name: "Configure Docker Daemon"
   template:
-    src: 'daemon.json'
+    src: 'daemon.json.jinja2'
     dest: '/etc/docker/daemon.json'
     owner: 'root'
     group: 'root'

templates/daemon.json → templates/daemon.json.jinja2

 {
+{% if docker_subnet %}
     "bip": "{{ docker_subnet }}"
+{% endif %}
 }