Do not set up acme-challenge forwarding if the acme service is not running

roles/nginx/templates/config/sites-available/default

@@ -2,11 +2,15 @@ server {
         listen [::]:80 default_server ipv6only=off;
         server_name _;
 
+{# Only enable this if the 'acme' service is defined,
+   or the basic tests will fail #}
+{% if 'acme' in services %}
         # Handle ACME challenge verification.
         location /.well-known/acme-challenge {
 		include /etc/nginx/snippets/proxy.conf;
 		proxy_pass http://acme.{{ domain }};
         }
+{% endif %}
 
         # Redirect everything else to HTTPS.
         location / {