create cnd to enforce hst:configuration structure

To make sure that all users who are going to develop with the hst stick to the logic we want to provide, I will make a cnd for the hst configuration forcing structure. Rather I have change requests by users/developers later on, and, if useful, add this to the cnd, then have a diverge set of hst configurations organically grown by developers being free to define any nodes and properties. This would namely be very hard to support by the cms, to provide an interface for configuring a hst interface.