Allow features to be patched at distribution assembly time

ODLPARENT-189 mostly resolved the issues we have with respect to standard/pax-web features. Unfortunately there is duplication in external features, which we do not pull into opendaylight-karaf-empty, which are not handled.

We need a mechanism by which odlparent can provide replacement features known to odlparent (such as hibernate-osgi), but are not part of the core required set.