use installMode="root" for top-level JBDS "Core Features" feature

To better support the BYOE scenario and be more easily able to update pieces of JBDS w/o having to update the product, we could consider adding `installMode="root"` to the JBDS "Core Features" feature so it can be installed in parallel to the JBDS "Branded Product" feature, rather than as its child [1].

[1] http://codeandme.blogspot.fr/2014/06/tycho-11-install-root-level-features.html

We could further do this with other JBDS/JBT child features, but I suspect we don't want to do that as it would allow users to update individual parts piecemeal, rather than as one big update.