Add an option to silence snmpit when building vlans; used from the protogeni code to cut down on the logging mail.
Add an option to silence snmpit when building vlans; used from the
protogeni code to cut down on the logging mail.

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