[task/config-class] Add an attribute for the table name in phpbb_config_db.

PHPBB3-9988

phpBB/includes/config/db.php

@@ -33,6 +33,12 @@ class phpbb_config_db extends phpbb_config
 	*/
 	protected $db;
 
+	/**
+	* Name of the database table used for configuration.
+	* @var string
+	*/
+	protected $table;
+
 	/**
 	* Creates a configuration container with a default set of values
 	*