roles/nginx: Allow custom resolvers for TLS stapling

Allows to specify custom DNS resolvers for TLS stapling, with a default
of Cloudflare's public DNS servers.

roles/nginx/defaults/main.yml

@@ -16,6 +16,10 @@ nginx_ssl_buffer_size: 1400
 nginx_ssl_dhparam: /etc/ssl/certs/dhparam.pem
 nginx_ssl_protocols: 'TLSv1 TLSv1.1 TLSv1.2'
 
+# DNS resolvers for OCSP stapling (default to Cloudflare public DNS)
+# See: https://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_stapling
+nginx_ssl_stapling_resolver: '1.1.1.1 1.0.0.1 [2606:4700:4700::1111] [2606:4700:4700::1001]'
+
 # install certbot + dependencies?
 # True unless you're in development and using "localhost" + snakeoil certs
 use_letsencrypt: True