openflow.nicira: Use custom OF message handlers
openflow.nicira: Use custom OF message handlers

The new OpenFlow message handler mechanism both broke the fairly
hacky way the nicira module used to hook vendor messages and also
allows for a more elegant solution.  This commit rectifies things.

Specifically, it creates a new handler subclass for switches with
Nicira extensions, which it enables in a ConnectionHandshakeComplete
handler.

Signed-off-by: Murphy McCauley <4a186f831f9e499082222e927502d182f9e495ec@gmail.com>

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