allow port to be 'UNSET'.

This allows to have a setup where rabbitmq is not listening
on an non ssl port.
eg:
  class { '::rabbitmq':
    ssl                      => true,
    ssl_cacert               => '/etc/rabbitmq/ssl/cacert.pem',
    ssl_cert                 => '/etc/rabbitmq/ssl/cert.pem',
    ssl_key                  => '/etc/rabbitmq/ssl/key.pem',
    config_variables         => { tcp_listeners => '[]' },
    port                     => 'UNSET',
  }