Updates ['rabbitmq']['config'] to use ['rabbitmq']['config_root'] attribute

Attribute ['rabbitmq']['config'] should not be hard coding again.
Cover the following two lines:
https://github.com/jjasghar/rabbitmq/blob/master/attributes/default.rb#L19 
https://github.com/jjasghar/rabbitmq/blob/master/attributes/default.rb#L89

Fixes issue #155