fixed system_services.sh

I needed to change `system_services.sh` in order to run 

    docker build image

successfully. The installation of `syslog-ng` failed because `apt-get install` will try to start the `syslog-ng` service before `system_services.sh` has the chance to modify the `syslog-ng` configuration.
Running `RUNLEVEL=1 apt-get install ...` will prevent this.