With shared builds, it's now possible that the [python2.7] part that is
providing executable for this buildout does not need to install any part and
makes the interpreter available since recipe initialisation.

This is causing an error during the overriden method computing part signature,
because parts are not installed yet:

    An internal error occurred due to a bug in either zc.buildout or in a
    recipe being used:
    Traceback (most recent call last):
      File "zc/buildout/buildout.py", line 2242, in main
        getattr(buildout, command)(args)
      File "zc/buildout/buildout.py", line 638, in install
        self._install_parts(install_args)
      File "zc/buildout/buildout.py", line 704, in _install_parts
        self._compute_part_signatures(install_parts)
      File "slapos/rebootstrap/__init__.py", line 37, in wrapper
        return getattr(self, attr)(*args, **kw)
      File "slapos/rebootstrap/__init__.py", line 95, in _compute_part_signatures
        installed_part_options[part]['__buildout_signature__']
    KeyError: 'already_installed_python'

To support that case, tolerate that parts are can be not installed yet at this
point.

/reviewed-on nexedi/slapos.rebootstrap!1

- 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.