- Work entirely in-place, even to switch to a different version of Python.
- Update bin/buildout to immediately use the wanted Python on subsequent
  buildout runs.

For SlapOS, the second point is required to have the instanciation done
with the built Python.

  Preserve bin-directory is required to while regenerate bin/buildout on
  the right location.

 * Same parts directory as the normal buildout is used so that we can
   build faster and also make the code simpler.
 * Python part is now specified by `python` parameter in `buildout`
   section.
 * Restart automatically with the original python when running python
   part is removed.

rebootstrap_section, relative_python and prefix are only used in __init__
method and there is no need to clutter object.