fix crash of custom tags when running against Mongo datastores. Mongo's 'system'…

fix crash of custom tags when running against Mongo datastores. Mongo's 'system' does not have a course_id attribute defined, which makes it asymmetric to XmlFilesystem store

common/lib/xmodule/xmodule/modulestore/mongo.py

@@ -51,6 +51,9 @@ class CachingDescriptorSystem(MakoDescriptorSystem):
         self.modulestore = modulestore
         self.module_data = module_data
         self.default_class = default_class
+        # cdodge: other Systems have a course_id attribute defined. To keep things consistent, let's
+        # define an attribute here as well, even though it's None
+        self.course_id = None
 
     def load_item(self, location):
         location = Location(location)