r/scrape-collectd: collectd scrape targets config

The *scrape-collectd* role generates the
`/etc/prometheus/scrape-collectd.yml` file.  This file can be read by
Prometheus/Victoria Metrics/vmagent to identify the hosts running
*collectd* with the *write_prometheus* plugin, using the
`files_sd_configs` scrape configuration option.

All hosts in the *collectd-prometheus* group are listed as scrape
targets.

roles/scrape-collectd/tasks/main.yml

@@ -0,0 +1,8 @@
+- name: ensure scrape hosts file is populated
+  copy:
+    dest: /etc/prometheus/scrape-collectd.yml
+    content: |+
+      {{ scrape_collectd_config | to_nice_yaml }}
+    mode: u=rw,go=r
+    owner: root
+    group: root

roles/scrape-collectd/vars/main.yml

@@ -0,0 +1,2 @@
+scrape_collectd_config:
+- targets: '{{ groups["collectd-prometheus"] }}'