vnc: Disable TLS encryption if server cert and key are not set

Some VNC clients, i.e. noVNC, do not support TLS authentication method. Add a possibility to use password-only VNC authentication. TLS is not activated if no server certificate and key are provided. Although this weakens the security level, it will allow to extend VNC clients compatibility, and can be used in some use-cases, like internal networks, where connection security is not strictly required. Update man page accordingly.

Signed-off-by: Lukasz Czechowski lukasz.czechowski@thaumatec.com