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/templates/weave.service.j2

@@ -1,7 +1,7 @@
 [Unit]
 Description=Weave Network
 Documentation=http://docs.weave.works/weave/latest_release/
-Wants=docker.service docker.socket
+Wants=docker.socket
 After=docker.service docker.socket
 
 [Service]