Comment 6 for bug 1621180

Verified with:

lxc launch ubuntu-daily:xenial sm-x0 "--config=user.user-data=$(cat /tmp/foo.ud)"
## get proposed version
lxc exec sm-x0 -- sh -c '
    p=/etc/apt/sources.list.d/proposed.list
    echo deb http://archive.ubuntu.com/ubuntu xenial-proposed main > "$p" &&
    apt-get update -q && apt-get -qy install cloud-init'

## cleanup
lxc exec sm-x0 -- sh -c '
    cd /var/lib/cloud && for d in *; do [ "$d" = "seed" ] || rm -Rf "$d"; done
    rm -Rf /var/log/cloud-init*'

lxc exec sm-x0 reboot
$ lxc exec sm-x0 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