lay down a systemd dropin instead of the /run/flannel_docker_opts.env symlink

roles/network_plugin/flannel/tasks/main.yml

@@ -42,12 +42,14 @@
     mode: 0644
   notify:
     - restart docker
+  when: ansible_os_family != "CoreOS"
 
-- name: Flannel | Create docker config symlink for CoreOS
-  file:
-    src: "/etc/default/docker"
-    dest: "/run/flannel_docker_opts.env"
-    state: link
+- name: Flannel | Create docker dropin for CoreOS
+  template:
+    src: docker-dropin
+    dest: "/etc/systemd/system/docker.service.d/flannel-options.conf"
+  notify:
+    - restart docker
   when: ansible_os_family == "CoreOS"
 
 - meta: flush_handlers

roles/network_plugin/flannel/templates/docker-systemd

+[Service]
+Environment="DOCKER_OPTS=--bip={{ flannel_subnet }} --mtu={{ flannel_mtu }} {% if docker_options is defined %}{{ docker_options }}{% endif %}"