Add openscap tooling and implement scan of images

We can improve the building process by adding openscap in the existing tooling.

After a successful build and the boot validation phases, we can add a new CI task doing:

• Starting the build image with qemu with cloud-init drive to inject some extra conf for openscap
• Invoking openscap tooling to start the analysis
• Save the result as artifacts or something else which can be injected in the release file.