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/weave/tasks/main.yml

@@ -7,14 +7,7 @@
     group: root
     mode: 0644
   notify:
-    - Weave | restart docker
-
-- name: Write docker.service systemd file
-  template:
-    src: systemd-docker.service
-    dest: /lib/systemd/system/docker.service
-  notify: Weave | restart docker
-  when: ansible_service_mgr == "systemd" and ansible_os_family != "CoreOS"
+    - restart docker
 
 - name: Weave | Install weave
   command: rsync -piu "{{ local_release_dir }}/weave/bin/weave" "{{ bin_dir }}/weave"