As far as I can see, unscd is explicitly listed as needed package:
$ grep -rI unscd
build-openstack-debian-image:
NEEDED_PACKAGES=${NEEDED_PACKAGES},python,python-paramiko,python-argparse,python-cheetah,python-configobj,python-oauth,python-software-properties,python-yaml,python-boto,python-prettytable,initramfs-tools,python-requests,acpid,acpi-support-base,ntp,unscd
build-openstack-debian-image:
NEEDED_PACKAGES=${NEEDED_PACKAGES},cloud-init,cloud-utils,cloud-initramfs-growroot,dbus,ntp,unscd
openstack-debian-images-1.20 as fetched by apt-get source
