Fix LocalAgent existance check during import

When importing a LocalAgent, the backup tool checks if the agent already exist in the database (to determine whether it should insert or update it). But the check is incorrect, because it does not take into account the owner of the LocalAgent when checking if it exists. In practice, this means if another gateway instance (which shares the database) has a LocalAgent with the same name, the backup tool will try to update that agent, instead of inserting a new one.