[RFE] Enable composer to point to existing parent commit

[apitanga]

Users should be able to generate new Edge images, as well as image updates which require composer to point to a "parent commit" during image creation

I believe it would require a change here:

rhel-edge-automation-arch/ansible/roles/build-rpm-ostree/tasks/main.yaml

Line 54 in 2fb1d30

composer-cli -j compose start {{ blueprint_name}} rhel-edge-commit

The idea would be to only pass the variables {{ ref }} and {{ parent }} if needed

[apitanga]

CLi procedure would be something like: composer-cli compose start-ostree --parent faf281d82fa6c9bd0671511a6043f6fcf8a592cae265a3d27cb9f8379cb5dd98 edge rhel-edge-commit