Have clear and flexible support for multiple interfaces.

[jheretic]

Right now we just assume one interface of each type. However, in your average router, we might have many different physical and virtual interfaces. We should evaluate whether this is a use case we want to manage in QuickStart, and how we might present that, or if we just want to handle it post-QuickStart.

[seamustuohy]

closed per opentechinstitute/luci-commotion#110