I don't see any reason why we don't use upstream zope *versions.cfg files. This
just duplicates work and makes maintenance tedious especially now that we have
zope2, zope4py2 and zope4py3.

Also:
  * Remove pinned versions on eggs with newer version or already defined in
    Zope versions.cfg files.
  * Make sure that pinned versions in stack/slapos.cfg do not override Zope versions
    (for example ZConfig versions set in stack/slapos.cfg was creating a version
    conflict while installing another egg).