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/handlers/main.yml

@@ -1,10 +1,4 @@
 ---
-- name: Weave | restart docker
-  command: /bin/true
-  notify:
-    - Weave | reload systemd
-    - Weave | reload docker
-
 - name: restart weave
   command: /bin/true
   notify:
@@ -27,11 +21,6 @@
     - Weave | reload systemd
     - reload weaveexpose
 
-- name: Weave | reload docker
-  service:
-    name: docker
-    state: restarted
-
 - name: reload weave
   service:
     name: weave