Fix column variable typo

attach_datetime is not available during downgrade. It's a copy/paste
bug from the upgrade function.

Partially fix bug 1037183

Change-Id: I97e2cf98a180a6c80e59545a9cda9852fa6ac563

nova/db/sqlalchemy/migrate_repo/versions/098_update_volume_attach_time.py

@@ -64,7 +64,7 @@ def downgrade(migrate_engine):
                 where(volumes.c.id == v['id']).\
                 values(attach_string=attach_time).execute()
     except Exception:
-        attach_datetime.drop()
+        attach_string.drop()
         raise
 
     old_attachtime.alter(name='attach_time_old')