`rebuild_base_installers.py` should focus on building

[aaaaalbert]

rebuild_base_installers.py currently populates and packages (tars, gzips or zips) base installer directories for the Custom Installer Builder. However, it does other things that might be out of scope as they are tied closely to the layout and config of the CIB: It copies tarballs to the deployment dir, backups previous base installers, chowns files, etc.

All of these things are important for deploying a set of new base installers, none of them are actual packaging tasks however. I think we should remove the unrelated parts.

(However, it will probably make sense to still include checking for the softwareupdater URL and key etc., just so that CIB operators don't shoot themselves in the foot).