r/collectd-prometheus: Allow scraping from outside

The *collectd-prometheus* role now has a
`collectd_prometheus_allow_outsize` variable.  This variable controls
whether or not external hosts are allowed to scrape data from *collectd*.
When set to `false`, as is the default value, *collectd* will be
configured to listen on the loopback interface only, and the TCP port
will not be opened in the firewall.

roles/collectd-prometheus/defaults/main.yml

@@ -1,2 +1,5 @@
 collectd_load_prometheus_plugin: true
 collectd_prometheus_port: 9103
+collectd_prometheus_allow_outside: false
+collectd_prometheus_host: >-
+  {% if not collectd_prometheus_allow_outside %}::1{% endif %}