
Add call "plabeventkeys" for listing all event system keys for the
experiments on a particular plab pnode.  Can only be called by the
service slice.

Returns tuples, one per line, for each experiment on the node like so:

PID=<pid> EID=<eid> KEY=<eventsys_key>

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome