tools/iotlab-testbed: handle IOTLAB_NODE in comma separated form
tools/iotlab-testbed: handle IOTLAB_NODE in comma separated form

authmillenon/RIOT,OlegHahm/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,OTAkeys/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,miri64/RIOT,OTAkeys/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,jasonatran/RIOT