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/defaults/main.yml

@@ -6,3 +6,5 @@ collectd_network_listen: []
 collectd_disable_plugins: []
 
 collectd_df: {}
+
+collectd_load_unixsock_plugin: true