Make device-init run after docker service in systemd

[marcosnils]

https://github.com/hypriot/device-init/blob/master/package/etc/systemd/system/device-init.service#L4

Current systemd configuration runs device-init after network but doesn't wait for docker to be ready. I'm not sure how it's working and I might be confused but I haven't seen anywhere in the code that device-init waits for the daemon to be up and ready to start loading the images.