add template conditional around the pihole exporter (#447)

Author: mjperrone

templates/prometheus.yml.j2

@@ -72,10 +72,11 @@ scrape_configs:
   - job_name: 'node'
     static_configs:
       - targets: {{ prometheus_node_exporter_targets | to_yaml }}
-
+  {% if pihole_enable %}
   - job_name: 'pihole-exporter'
     static_configs:
       - targets: ['pihole:9617']
+  {% endif %}
 
 {% filter indent(width=2,first=True) %}
 {{ prometheus_extra_scrape_configs }}