Slightly change to the strategy of overwriting bootstrap variables
* the theme`s variables.less file will now contain all bootstrap variables as they are, merged with your custom overwrites.
* The original bootstrap variables.less file will no longer be needed and will not be imported to your theme`s bootstrap.less anymore
Refactor configuration paths for bootstrap
* Remove root bootstrap path from configuration
* Add bootstrap configuration paths for less, js and assets
* Suffix holder and jquery assets with "_js"
* Fix unit tests