This is used to start an ostree build, it is only supported on
ostree-composer, and requires the ostree ref and parent. It may also
include --size and optionally be uploaded.
osbuild-composer can support user specified image sizes, this adds an
optional argument, after start, to specify the size in MiB. eg.
composer-cli compose start --size 2048 example-http-server qcow2
This only works when the backend is not 'lorax-composer', when it is the
user will get a warning that it will be ignored.
This patch does two things:
1) Add "compose list", which lists compose UUIDs and other basic info,
2) Fix up "blueprints list", "modules list", "sources list", and
"compose types" so their output is just a plain list of identifiers
