This will allow nested slapos where the inner slapos can re-use shared
parts from outer slapos, but not write into the outer shared parts.

For compatibility reasons, the syntax of the buildout option changed,
instead of the old ${buildout:shared-parts} that use to be one path, we
now use ${buildout:shared-part-list} as a \n separated list of paths.
All paths can be used to lookup existing parts, only the last one can be
used to install new parts.
There is no compatibility with the old option as it was not used much,
only in profiles which were not really compatible with shared options.