Remove uses of using() from migrations

This hardcoded the db_alias fetched from schema_editor and forces django
to try and migrate any second database you use, rather than routing to
the default database.  In testing a build from scratch, these do not
appear needed.

Using using() prevents us from using multiple databases behind edxapp.

Additionally - add back a removed backwards migration from certificates
0003.  I have no idea why this was dropped in the 1.8 upgrade.

common/djangoapps/embargo/migrations/0002_data__add_countries.py

@@ -10,15 +10,13 @@ from django_countries import countries
 def create_embargo_countries(apps, schema_editor):
     """Populate the available countries with all 2-character ISO country codes. """
     country_model = apps.get_model("embargo", "Country")
-    db_alias = schema_editor.connection.alias
     for country_code, __ in list(countries):
-        country_model.objects.using(db_alias).get_or_create(country=country_code)
+        country_model.objects.get_or_create(country=country_code)
 
 def remove_embargo_countries(apps, schema_editor):
     """Clear all available countries. """
     country_model = apps.get_model("embargo", "Country")
-    db_alias = schema_editor.connection.alias
-    country_model.objects.using(db_alias).all().delete()
+    country_model.objects.all().delete()
 
 class Migration(migrations.Migration):