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/meta/main.yml

@@ -6,3 +6,4 @@ dependencies:
    when: kube_network_plugin == 'flannel'
  - role: network_plugin/weave
    when: kube_network_plugin == 'weave'
+ - role: docker