Introduce dns_mode and resolvconf_mode and implement docker_dns mode

Also update reset.yml to do more dns/network related cleanup.

roles/docker/tasks/systemd.yml

@@ -21,4 +21,11 @@
     dest: "/etc/systemd/system/docker.service.d/docker-options.conf"
   notify: restart docker
 
+- name: Write docker dns systemd drop-in
+  template:
+    src: docker-dns.conf.j2
+    dest: "/etc/systemd/system/docker.service.d/docker-dns.conf"
+  notify: restart docker
+  when: dns_mode != 'none' and resolvconf_mode == 'docker_dns'
+
 - meta: flush_handlers