Add rest client setting

This PR intentds in enable possibility to choose the rest driver to use on the REST BDD testing through `behat.yml` settings.

Also renames classes to be more clear that those aren't actually clients, but drivers to the actual clients

This needs ezsystems/ezpublish-community#176
Got it, adding default just like: [RestContext#L56](https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Bundle/EzPublishRestBundle/Features/Context/RestContext.php#L56)