Comment 8 for bug 1621180

verified as shown in template.
$ printf "#cloud-config\napt_mirror: ''\n" > user-data
$ n=x1
$ lxc delete --force $n
$ lxc launch ubuntu-daily:xenial $n
$ sleep 10
$ lxc exec $n -- sh -c '
    p=/etc/apt/sources.list.d/proposed.list
    echo deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc)-proposed main > "$p" &&
    apt-get update -q && apt-get -qy install cloud-init' </dev/null

$ lxc file push - $n/etc/cloud/cloud.cfg.d/mirror.cfg < user-data

## clean it out so next is first boot.
$ lxc exec $n -- sh -c '
  cd /var/lib/cloud && for d in *; do [ "$d" = "seed" ] || rm -Rf "$d"; done
  rm -Rf /var/log/cloud-init*'

$ lxc exec $n reboot
$ lxc exec $n -- tail -f /var/log/cloud-init-output.log
$ lxc exec $n grep ^deb /etc/apt/sources.list | head -n 3
deb http://archive.ubuntu.com/ubuntu xenial main restricted
deb-src http://archive.ubuntu.com/ubuntu xenial main restricted
deb http://archive.ubuntu.com/ubuntu xenial-updates main restricted

$ lxc exec $n -- dpkg-query --show cloud-init
cloud-init 0.7.8-1-g3705bb5-0ubuntu1~16.04.1