roles/nginx: Set nginx SSL buffer size to 1400

istlsfastyet.com recommends setting the buffer size to 1400 so it
can fit into a single MTU.  nginx default is 16k!

http://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_buffer_size

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/nginx/templates/https.j2

@@ -7,6 +7,7 @@
 
     ssl_session_timeout 5m;
     ssl_session_cache shared:SSL:1m;
+    ssl_buffer_size 1400; # 1400 bytes to fit in one MTU
     ssl_dhparam /etc/ssl/certs/dhparam.pem;
     ssl_protocols {{ nginx_tls_protocols }};
     ssl_ciphers "{{ tls_cipher_suite }}";