Fix to support nested carousels

To allow usage of nested carousels, there is a need to constraint selectors to only first level of elements so children carousels will be left untouched by sliding of parent carousel.

Related issues:
https://github.com/twitter/bootstrap/issues/4359 unable to put carousel inside carousel
https://github.com/twitter/bootstrap/issues/7734 Conflict on item divs using nested carousel

PS: Sorry if I format anything in wrong way, this is my second pull request (first was failure https://github.com/twitter/bootstrap/pull/8448).