Check zc.buildout version on import

Raise if it's a slapos-patched zc.buildout. This replaces having
a constraint in setup.py as setting zc.buildout>=3.0.1+slapos001
is invalid.

slapos/rebootstrap/__init__.py

@@ -17,6 +17,14 @@ import pkg_resources
 from zc.buildout import easy_install, UserError
 
 
+buildout_dist = pkg_resources.get_distribution('zc.buildout')
+if 'slapos' not in str(buildout_dist.version):
+  raise UserError(
+    "Incompatible version %s\n"
+    "Consider installing e.g. zc.buildout==3.0.1+slapos001"
+    % buildout_dist)
+
+
 class FakeSysExecutable(object):
 
   def __init__(self, python):