setup-testbed: properly get the main network interface

On Ubuntu's production deployment, with bionic LXD images, an issue arose because of an extra erspan0 interface being present for some reason. It's pretty clear that IFACE should only contain one main interface, so getting it from the default route is likely to yield the best result, even if there might be cases where this still breaks.

This is more or less the first solution I came up with, but I'm happy to discuss any other way of getting the main network interface in this MR.