Authenticates the user taking the Common Name of the client's SSL
certificate as the username. 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.

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.