Added support for cert-only login without user and password

Added new DialTLS_CertAuth function for RabbitMQ configured for EXTERNAL auth with ssl_cert_login plugin for userless/passwordless logons.

As of now, amqp just silently hangs, when server presents authentication metod not supported by amqp (when pickSASLMechanism finds no suitable match). I guess it should bail out with some sort of "No common SASL methods found" message.

Hope this helps,
 best regards.