Move generic buildout parts to common Plone buildout

[avoinea]

@yosifkit comment on docker-library/official-images#9770 (comment)

Just a note that buildout.cfg and docker-initialize.py seem like quite a lot to have outside the standard Plone packaging. Would it benefit users outside of Docker images to have all/part of these files in the regular Plone-*-UnifiedInstaller-1.0.tgz release?