testnode: make sure revision is not defined

We have local repository to control which repository at which revision is tested.

erp5/util/testnode/testnode.py

@@ -169,6 +169,7 @@ develop = false
 [%(buildout_section_id)s]
 repository = %(repository_path)s
 branch = %(branch)s
+revision =
 develop = false
 """ %     {'buildout_section_id': buildout_section_id,
           'repository_path' : repository_path,