Code review comment for ~raharper/netplan:fix-mtu

These tests should be run from within autopkgtest:

(from the netplan source tree root)
autopkgtest -U --apt-pocket=proposed -s --test-name integration.py . -- qemu ~/adt-zesty-amd64.img

You can build the necessary zesty qemu image for autpkgtests with:

adt-buildvm-ubuntu-cloud -r zesty -a amd64

This is important, my initial manual testing showed that neither networkd nor NM devices ended up having their MTU set; so we should still test both NM and networkd renderers to make sure that the MTU is set properly on the underlying devices when this runs, and to properly automate testing this behavior (given that it's reportedly playing with udev, so there are possibly other steps to take before the changes will really be applied).