r/collectd-version: Add OS version plugin

The `collectd-version` script uses the *collectd* UNIX socket to send
custom values to *collectd* to track the OS version.  Since these values
obviously cannot change while the system is running, the values are
specified with a very long interval.  This avoids having to continuously
insert the values, either with a long-running process or by repeatedly
running a script.  The values only need to be inserted once when
*collectd* starts.

roles/collectd-version/files/collectd-version.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=collectd version plugin
+BindsTo=collectd.service
+After=collectd.service
+
+[Service]
+ExecStart=/usr/local/libexec/collectd-version
+RemainAfterExit=true
+
+[Install]
+WantedBy=collectd.service