Implement the correct deletion restrictions in Route53 _process_delete

[exarkun]

txaws/testing/route53.py:

        # The hosted zone itself must always have an SOA and an NS.  It is an                                                                                                                                      
        # error to attempt to delete either of those.                                                                                                                                                              
        #                                                                                                                                                                                                          
        # However, there may be NS records for subdomains of the hosted zone                                                                                                                                       
        # (ie, delegations).  It's okay to delete those.  We're not quite                                                                                                                                          
        # smart enough to recognize that case here.