Preserve bin-diretory from the original buildout

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

slapos/rebootstrap/__init__.py

@@ -88,11 +88,14 @@ Buildout will be restarted automatically to have this change applied.
       extension_list = self.buildout['buildout']['extensions'].split()
       extension_list = [q.strip() for q in extension_list if q.strip() != \
           __name__]
+      bin_directory = self.buildout['buildout']['bin-directory']
       # rerun buildout with only neeeded section to reuse buildout
       # ability to calcuate all dependency
       args.extend([
         # chroot to rebootstrap directory
         'buildout:directory=%s' % self.rebootstrap_directory,
+        # preserve bin-directory outside the chroot.
+        'buildout:bin-directory=%s' % bin_directory,
         # install only required section with dependencies
         'buildout:parts=%s' % self.python_section,
         # do not load this extension