Move docker systemd unit creation to docker role

Creating the unit using default settings early on
and then changing it during network_plugin section
leads to too many docker restarts and duplicated code.

Reversed Wants= dependence on docker.service so it does not
restart docker when reloading systemd

Consolidated all docker restart handlers.

roles/network_plugin/flannel/tasks/main.yml

@@ -50,11 +50,4 @@
     state: link
   when: ansible_os_family == "CoreOS"
 
-- name: Flannel | Write docker.service systemd file
-  template:
-    src: systemd-docker.service
-    dest: /lib/systemd/system/docker.service
-  notify: restart docker
-  when: ansible_service_mgr == "systemd" and ansible_os_family != "CoreOS"
-
 - meta: flush_handlers