functionality for choosing a different database connection.

Added new functionality to use a different database connection with the virtuser_query plugin. Currently the plugin can only use the default roundcube connection. 

You need to add a parameter to the main.inc.php:
//virtuser_query_dsn if not set the default dsn will be taken.
$rcmail_config['virtuser_query_dsn'] = 'mysql://roundcube:password@dbhost/database';

And this database will be connected to executed the queries.

If this option is not given the default dbconnection will be used (as it was before).