roles/collectd-prometheus: Configure plugin

The *collectd-prometheus* role configures the *write_prometheus* plugin
for collectd.  This plugin exposes data collected or received by the
collectd process in the Prometheus Exposition Format over HTTP.  It
provides the same functionality as the "official" collectd Exporter
maintained by the Prometheus team, but integrates natively into the
collectd process, and is much more complete.

The main intent of this role is to provide a mechanism to combine the
collectd data from all Pyrocufflink hosts and insert it into Prometheus.
By configuring the collectd instance on the Prometheus server itself to
enable and use the *write_prometheus* plugin and to receive the
multicast data from other hosts, collectd itself provides the desired
functionality.

roles/collectd-prometheus/defaults/main.yml

@@ -0,0 +1,2 @@
+collectd_load_prometheus_plugin: true
+collectd_prometheus_port: 9103