Extra flag to specify directive ssl_verify_client

[stainboy]

I would suggest to provide an extra flag CertificateAuth.VerifyClientFlag to specify nginx directive ssl_verify_client. And leave the default value to on to keep the compatibility. In our use case, client certificate authentication is preferred but not required. If it fails, FORM authentication will be provided as well.

ssl_verify_client = on | off | optional | optional_no_ca;

https://github.com/kubernetes/ingress/blob/b2ad9e7078a1ab0f843ec7962b80703efd15e978/controllers/nginx/rootfs/etc/nginx/template/nginx.tmpl#L586