Make it possible to disable umask by providing an empty value.

Useful when extending another section from same file.

slapos/recipe/template/jinja2_template.py

@@ -87,6 +87,9 @@ class Recipe(object):
                 expression, buildout, name, options)
         if 'umask' in options:
             self.umask = int(options['umask'], 8)
+        umask_value = options.get('umask')
+        if umask_value:
+            self.umask = int(umask_value, 8)
         else:
             self.umask = os.umask(0)
             os.umask(self.umask)