customize_cloud_init could fail gracefully when cloud-init not present

Hi,

While switching an image definition using cloud-init from server-minimal to desktop-minimal, I had this error:
ubuntu-images$ sudo /snap/bin/ubuntu-image classic ubuntu-desktop-tegra-igx-ppa.yaml
[sudo] password for lool:
[0] parse_image_definition
[1] calculate_states
[2] make_temporary_directories
[3] determine_output_directory
[4] build_gadget_tree
[5] prepare_gadget_tree
[6] load_gadget_yaml
WARNING: volumes:pc:structure:0:filesystem_label used for defining partition roles; use role instead
[7] verify_artifact_names
[8] germinate
[9] create_chroot
[10] add_extra_ppas
[11] install_packages
[12] prepare_image
[13] preseed_image
[14] customize_cloud_init
Error: open /tmp/ubuntu-image-89148fee-82a2-4338-9bd2-fa75c7371e6f/chroot/etc/cloud/cloud.cfg.d/90_dpkg.cfg: no such file or directory

This was due to cloud-init not being pulled by the new set of seeds.

It would be nice if ubuntu-image could simply output a warning / error if cloud-init customizations are requested but cloud-init is not present, rather than fail with a cryptic error.

This was with latest edge snap, 3.0+snap10 (555).

Thanks!