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

Wrt. autopkgtest invocation there's a slight optimization: If you don't want autopkgtest to waste time on building the binary packages first but you already built them (in schroot or whereever), you can add them to the command line. Also, I'd suggest to not test against -proposed locally -- it's not what happens in production CI either and only slows down local iterations:

$ autopkgtest -s --test-name integration.py . /tmp/build-area/*.deb -- qemu ~/adt-zesty-amd64.img