[reboot] stop docker service before rebooting (sonic-net#423)

* [reboot] stop docker service before rebooting

After upgrading to docker-ce 18.09.0. Shutdown SONiC takes 5-10 minutes
waiting for all the running docker containers to quit. They all enventually
hit the timeout and caused the huge delay.

This is a work-around until proper solution is found.

Signed-off-by: Ying Xie <ying.xie@microsoft.com>

* [docker] always stop docker on all platforms

- add a protection of 30 seconds timeout.

Signed-off-by: Ying Xie <ying.xie@microsoft.com>

scripts/reboot

@@ -15,6 +15,10 @@ function stop_sonic_services()
         docker exec -i syncd /usr/bin/syncd_request_shutdown --cold > /dev/null
         sleep 3
     fi
+
+    # Stop docker service to avoid 5-10 minutes delay in shutdown
+    # while waiting for existing docker containers to quit.
+    timeout 30s systemctl stop docker
 }
 
 function clear_warm_boot()