Comment 4 for bug 1563487

This seems fix-released for xenial images:
$ wget http://cloud-images.ubuntu.com/releases/xenial/release-20160420.3/ubuntu-16.04-server-cloudimg-amd64-disk1.img
$ sudo mount-image-callback --read-only --cd ubuntu-16.04-server-cloudimg-amd64-disk1.img -- sh -xc 'cat etc/cloud/build.info; ls -l etc/network/interfaces.d/eth0.cfg; ls -l etc/network/interfaces.d/'
mount: /dev/nbd0p1 is write-protected, mounting read-only
+ cat etc/cloud/build.info
build_name: server
serial: 20160420-145324
+ ls -l etc/network/interfaces.d/eth0.cfg
ls: cannot access 'etc/network/interfaces.d/eth0.cfg': No such file or directory
+ ls -l etc/network/interfaces.d/
total 0