role service-nginx: only provide location /metrics if node_exporter is availible

roles/service-nginx/templates/default.conf.j2

@@ -36,6 +36,7 @@ server {
         autoindex_exact_size off;
     }
 
+{% if node_exporter_present is defined %}
     location ^~ /metrics {
         set $metric_addr 127.0.0.1;
         proxy_pass http://$metric_addr:9100/metrics;
@@ -47,5 +48,6 @@ server {
         allow {{ lookup('dig', host, 'qtype=AAAA') }};
         deny all;
 {% endfor %}
+{% endif %}
     }
 }