a method to retrieve a list of server certificates

        :return: list with certificate name strings

def list_roles(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 iam roles.')
        try:
            response = self.connection.list_roles()
        except:
            raise AWSConnectionError(title)

    # construct certificate list from response
        role_list = []
        if 'Roles' in response.keys():
            for role in response['Roles']:
                role_list.append(role['RoleName'])

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

        self.role_list = role_list
        
        return self.role_list