Move cleanups to container-build-helpers

[dirkmueller]

We have a log of custom cleanups that need to be applied after zypper is run. rather than open-coding them in every container description, which means we have a lot of unnecessary churn on the container sources, we can move them into container-build-helpers and just call cleanup_container instead.

[alexandrevicenzi]

How late in the stage this is done? Because it appears that the log cleanup is not working for the busybox and init images (some backup files are still there), and I can't figure out why, if there's something running after we clean it in the config.sh script.