Authenticates the user, obtaining the username from the client's

 SSL certificate. The user's password is not checked.

 In order to use this mechanism the client must connect over SSL, and

 present a client certificate.

 For safety the server must be configured with the SSL options

 fail_if_no_peer_cert set to true and verify set to verify_peer, to

 force all SSL clients to have a verifiable client certificate.

 By default this will set the username to an RFC4514-ish string form of

 the certificate's subject's Distinguished Name, similar to that

 produced by OpenSSL's "-nameopt RFC2253" option.

 You can obtain this string form from a certificate with a command like:

 $ openssl x509 -in path/to/cert.pem -nameopt RFC2253 -subject -noout

 or from an existing amqps connection with commands like:

 $ rabbitmqctl list_connections peer_cert_subject

 To use the Common Name instead, set:

 {rabbitmq_auth_mechanism_ssl, [{name_from, common_name}]}

 in your configuration.

 Note that the authenticated user will then be looked up in the

 configured authentication / authorisation backend(s) - this will be

 the mnesia-based user database by default, but could include other

 backends if so configured.