adding checks to ensure directories exist
adding checks to ensure directories exist

This is in case the user has not run ./deps.sh prior to running the
mininet setup script.

netsec-ethz/scion,dmpiergiacomo/scion,dmpiergiacomo/scion,FR4NK-W/osourced-scion,netsec-ethz/scion,FR4NK-W/osourced-scion,klausman/scion,klausman/scion,FR4NK-W/osourced-scion,netsec-ethz/scion,dmpiergiacomo/scion,dmpiergiacomo/scion,dmpiergiacomo/scion,Oncilla/scion,Oncilla/scion,klausman/scion,Oncilla/scion,FR4NK-W/osourced-scion,klausman/scion,dmpiergiacomo/scion,Oncilla/scion,klausman/scion,FR4NK-W/osourced-scion,netsec-ethz/scion