r/collectd: Better control over enabled plugins

Instead of a simple list of disabled plugins, hosts and host groups can
now control whether plugins are enabled or disabled using the
`collectd_plugins` map.  The map keys are plugin names, and the values
are booleans indicating if the plugin is enabled.

Using this mechanism, some plugins can be disabled by default (e.g. the
*md* plugin), and enabling them per host or per host group is simpler.

roles/collectd/defaults/main.yml

@@ -3,7 +3,10 @@ collectd_load_network_plugin: false
 collectd_network_servers: []
 collectd_network_listen: []
 
-collectd_disable_plugins: []
+collectd_plugins: {}
+
+collectd_plugins_defaults:
+  md: false
 
 collectd_df: {}