Extend generate-pkg-centric-jobs.py to also generate image pipeline

The image building jobs are made dependencies of test jobs in order to
allow them to use freshly build image artifacts. The dependency is
marked optional to allow the pipeline to run with daily images as
before when build-custom-image: yes is not set for the package.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>