The stock Ubuntu 16 networking.service only runs `udevadm settle` if
there are 'auto ...' stanzas in /etc/network/interfaces .  Well, we got
rid of that a few commits ago, and now let udev rules populate
/etc/network/interfaces (really /run/emulab-interfaces.d-auto-added/*).
So, it's either hack the networking.service unit file to force udev to
settle, and have it blown away on package update; or add a
networking-emulab.service that has to run before networking.service to
force udev to settle.  We *always* want udev to settle on any Emulab
node before bringing up interfaces, just in case the control net NIC is
slow for whatever reason.