openflow: Log aborted connections at DEBUG
openflow: Log aborted connections at DEBUG

If we never even get a DPID from a switch, we now log its disconnection
at DEBUG level instead of INFO.  We never logged a connect message at
INFO in this case anyway, and it's not uncommon to have fairly meaningless
disconnection messages like this anyway, particularly with Mininet since
it probes the controller to see if it's listening.

pthien92/sdn,MurphyMc/pox,noxrepo/pox,noxrepo/pox,pthien92/sdn,MurphyMc/pox,MurphyMc/pox,pthien92/sdn,MurphyMc/pox,pthien92/sdn,pthien92/sdn,noxrepo/pox,noxrepo/pox,MurphyMc/pox