Add veth_interfaces table for configuring new veth virtual interfaces.
Unlike the interfaces table, veth interfaces come and go with experiment swapin and swapout, with assignment to be done in assign_wrapper. However, each veth maps to a real interface on a node, as defined in the interfaces table, and the cross link is the "iface" slot. There is a "mac" address since veth devices need a MAC, but I'm not sure how that will be assigned yet. I am letting the DB do the allocation of veth ids by making it an auto_increment field, and a dual key of (node_id, veth_id). So, the DB will assign increasing values per node_id, not globally to the table. Very slick.