Add support for HTTP/2 in Nginx

This is currently supported by more than 99% of the browsers, while
browsers not supporting it will fall back to HTTP/1.

With HTTP/2, browsers don't have to open more connections in order to
request more files, meaning that the page will load faster, particularly
when the page requires downloading many CSS files, JavaScript files or
images.

roles/nginx/templates/consul_vhost.j2

@@ -20,8 +20,8 @@ server {
 
   {% if domain is defined %}
 
-    listen [::]:443 ssl ipv6only=on;
-    listen 443 ssl;
+    listen [::]:443 ssl http2 ipv6only=on;
+    listen 443 ssl http2;
     ssl_certificate /etc/letsencrypt/live/{{ server_hostname }}/fullchain.pem;
     ssl_certificate_key /etc/letsencrypt/live/{{ server_hostname }}/privkey.pem;
     include /etc/letsencrypt/options-ssl-nginx.conf;