[TNL-5001] Import-export issues when importing legacy discussion OLX format

Observed Problems:
1. Discussion categories and targets were missing
2. When imported over existing course Discussion IDs have changed unexpectedly - all posts were missing

Solutions:
* Parsing legacy discussion OLX
* Do not force exporting discussion ID

common/lib/xmodule/xmodule/modulestore/tests/test_cross_modulestore_import_export.py

@@ -121,6 +121,9 @@ class CrossStoreXMLRoundtrip(CourseComparisonTest, PartitionTestCase):
                         self.exclude_field(None, 'wiki_slug')
                         self.exclude_field(None, 'xml_attributes')
                         self.exclude_field(None, 'parent')
+                        # discussion_ids are auto-generated based on usage_id, so they should change across
+                        # modulestores - see TNL-5001
+                        self.exclude_field(None, 'discussion_id')
                         self.ignore_asset_key('_id')
                         self.ignore_asset_key('uploadDate')
                         self.ignore_asset_key('content_son')

common/lib/xmodule/xmodule/xml_module.py

@@ -335,7 +335,7 @@ class XmlParserMixin(object):
 
         """
         # VS[compat] -- just have the url_name lookup, once translation is done
-        url_name = node.get('url_name', node.get('slug'))
+        url_name = cls._get_url_name(node)
         def_id = id_generator.create_definition(node.tag, url_name)
         usage_id = id_generator.create_usage(def_id)
         aside_children = []
@@ -344,8 +344,7 @@ class XmlParserMixin(object):
         if is_pointer_tag(node):
             # new style:
             # read the actual definition file--named using url_name.replace(':','/')
-            filepath = cls._format_filepath(node.tag, name_to_pathname(url_name))
-            definition_xml = cls.load_file(filepath, runtime.resources_fs, def_id)
+            definition_xml, filepath = cls.load_definition_xml(node, runtime, def_id)
             aside_children = runtime.parse_asides(definition_xml, def_id, usage_id, id_generator)
         else:
             filepath = None
@@ -408,6 +407,23 @@ class XmlParserMixin(object):
 
         return xblock
 
+    @classmethod
+    def _get_url_name(cls, node):
+        """
+        Reads url_name attribute from the node
+        """
+        return node.get('url_name', node.get('slug'))
+
+    @classmethod
+    def load_definition_xml(cls, node, runtime, def_id):
+        """
+        Loads definition_xml stored in a dedicated file
+        """
+        url_name = cls._get_url_name(node)
+        filepath = cls._format_filepath(node.tag, name_to_pathname(url_name))
+        definition_xml = cls.load_file(filepath, runtime.resources_fs, def_id)
+        return definition_xml, filepath
+
     @classmethod
     def _format_filepath(cls, category, name):
         return u'{category}/{name}.{ext}'.format(category=category,