Use go-systemd const instead of magic string in Linux version of dockerd

Signed-off-by: Christian Muehlhaeuser <muesli@gmail.com>

cmd/dockerd/daemon_linux.go

@@ -9,5 +9,5 @@ func preNotifySystem() {
 // notifySystem sends a message to the host when the server is ready to be used
 func notifySystem() {
 	// Tell the init daemon we are accepting requests
-	go systemdDaemon.SdNotify(false, "READY=1")
+	go systemdDaemon.SdNotify(false, systemdDaemon.SdNotifyReady)
 }