roles/nginx: add nginx ssl_ecdh_curve

This seems to be new since I last looked at the Mozilla server-side
SSL configurator.

roles/nginx/templates/https.j2

@@ -27,6 +27,7 @@
 
     ssl_dhparam {{ nginx_ssl_dhparam }};
     ssl_protocols {{ nginx_ssl_protocols }};
+    ssl_ecdh_curve {{ nginx_ssl_ecdh_curve }};
     ssl_ciphers "{{ tls_cipher_suite }}";
     ssl_prefer_server_ciphers on;