Add support for slapos.buildout >= 2.7.1+slapos004

When bootstrapping to a different version of Python, the path of
the wanted Python changes twice when using SlapOS shared parts.
This commit checks bin/buildout-rebootstrap before reusing it.

slapos/rebootstrap/__init__.py

@@ -12,7 +12,7 @@
 #
 ##############################################################################
 
-import logging, os, shutil, subprocess, sys, tempfile
+import errno, logging, os, shutil, subprocess, sys, tempfile
 from zc.buildout import easy_install, UserError
 
 
@@ -65,7 +65,14 @@ Buildout will be restarted automatically to have this change applied.
 
     installed = sys.argv[0]
     new_bin = installed + '-rebootstrap'
-    if not os.path.exists(new_bin):
+    try:
+      with open(new_bin) as x:
+        x = x.readline().rstrip()
+    except IOError as e:
+      if e.errno != errno.ENOENT:
+        raise
+      x = None
+    if x != '#!' + self.wanted_python:
       from .bootstrap import get_distributions, setup_script
       if subprocess.call((self.wanted_python, '-c',
           'import sys; sys.exit(sys.version_info[:2] == %r)'