Add interfaces to the ops_interfaces table on-demand.
Add interfaces to the ops_interfaces table on-demand.

We can't add all the interfaces all the time (since we have thousands
of them, and we're frequently repopulating the monitoring database), and
we can't statically determine the set we care about, either (since it
depends which VLANs we report as externally visible).

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