Cleanup `scion.sh sciond` (#876)
Cleanup `scion.sh sciond` (#876)

`./scion.sh sciond` now:
- takes a parameter in the standard ISD-AS form
- prints an error if no ISD-AS is specified, or if the topology dir
  doesn't exist.
- prints a message to say that sciond is running.
dmpiergiacomo/scion,dmpiergiacomo/scion,netsec-ethz/scion,Oncilla/scion,klausman/scion,klausman/scion,dmpiergiacomo/scion,Oncilla/scion,FR4NK-W/osourced-scion,FR4NK-W/osourced-scion,netsec-ethz/scion,FR4NK-W/osourced-scion,netsec-ethz/scion,klausman/scion,dmpiergiacomo/scion,Oncilla/scion,dmpiergiacomo/scion,klausman/scion,FR4NK-W/osourced-scion,FR4NK-W/osourced-scion,klausman/scion,netsec-ethz/scion,dmpiergiacomo/scion,Oncilla/scion