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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validate_rmq_ssl_enabled_units(self, sentry_units, port=None):
 """Check that ssl is enabled on rmq juju sentry units. :param sentry_units: list of all rmq sentry units :param port: optional ssl port override to validate :returns: None if successful, otherwise return error message """

        for sentry_unit in sentry_units:
            if not self.rmq_ssl_is_enabled_on_unit(sentry_unit, port=port):
                return ('Unexpected condition:  ssl is disabled on unit '
                        '({})'.format(sentry_unit.info['unit_name']))
        return None