add an option to exclude site-packages
We need an option in buildout that can be used to run the buildout without including site-packages.
With the Grok project we now advise people to create the buildouts inside a virtualenv which is installed with --no-site-packages. This to avoid the frequent problem that Python library installed by the platform conflicts with a Python library that Grok itself wants to install. On Mac OS X in particular this problem is guaranteed.
This complicates the installation instructions significantly. buildout is so close: buildout is all about controlling the environment, but always pulls in site-packages, an uncontrolled area.