mod_groups: Use module API for reading config

plugins/mod_groups.lua

@@ -80,7 +80,7 @@ function remove_virtual_contacts(username, host, datastore, data)
 end
 
 function module.load()
-	groups_file = config.get(module:get_host(), "core", "groups_file");
+	groups_file = module:get_option_string("groups_file");
 	if not groups_file then return; end
 	
 	module:hook("roster-load", inject_roster_contacts);