r/collectd: Enable unixsock plugin

The *unixsock* plugin for *collectd* provides a socket-based interface
that other software can use to communicate with *collectd*.  Notably,
this can be used to publish custom values, query existing values, and
flush caches.

The socket is created at `/run/collectd/socket`.  The `/run/collectd`
directory is managed by systemd; it will be created automatically when
the service starts and cleaned up when it stops.

roles/collectd/handlers/main.yml

@@ -1,3 +1,8 @@
+- name: reload systemd
+  systemd:
+    daemon_reload: true
+  changed_when: true
+
 - name: restart collectd
   service:
     name: collectd