collectd: Add collectd_df variable

The `collectd_df` variable can be used to configure the *df* plugin for
collectd.  It should contain a map on key-value pairs that correspond
exactly to the plugin's configuration options.

roles/collectd/defaults/main.yml

@@ -5,3 +5,5 @@ collectd_network_servers:
 collectd_network_listen: []
 
 collectd_disable_plugins: []
+
+collectd_df: {}