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/templates/collectd-unixsock.conf.j2

@@ -0,0 +1,18 @@
+{% if collectd_load_unixsock_plugin %}
+LoadPlugin unixsock
+{% endif %}
+
+<Plugin unixsock>
+    SocketFile "/run/collectd/socket"
+{% if collectd_unixsock_group|d %}
+    SocketGroup "{{ collectd_unixsock_group }}"
+{% endif %}
+{% if collectd_unixsock_perms|d %}
+    SocketPerms "{{ collectd_unixsock_perms }}"
+{% endif %}
+{% if collectd_unixsock_delete|d is true %}
+    DeleteSocket true
+{% elif collectd_unix_sock_delete|d is false%}
+    DeleteSocket false
+{% endif %}
+</Plugin>