a method to retrieve a list of server certificates

        :return: list with certificate name strings

def list_certificates(self):

        '''
            a method to retrieve a list of server certificates

        :return: list with certificate name strings
        '''

        title = '%s.list_certificates' % self.__class__.__name__

    # send request for list of certificates
        self.printer('Querying AWS for server certificates.')
        try:
            response = self.connection.list_server_certificates()
        except:
            raise AWSConnectionError(title)

    # construct certificate list from response
        cert_list = []
        if 'ServerCertificateMetadataList' in response.keys():
            for certificate in response['ServerCertificateMetadataList']:
                cert_list.append(certificate['ServerCertificateName'])

    # report results and return list
        if cert_list:
            print_out = 'Found server certificate'
            if len(cert_list) > 1:
                print_out += 's'
            from labpack.parsing.grammar import join_words
            print_out += ' %s.' % join_words(cert_list)
            self.printer(print_out)
        else:
            self.printer('No server certificates found.')

        self.certificate_list = cert_list
        
        return self.certificate_list