fix for descriptor element; refactoring

@miwest929 @sheavalentine-mdsol Please review and merge. Thanks!

The key here is registry_key method.
You can reference items like:

- href: name
and 
- href: DRDs#name <- assuming DRDs is external document

So, now you can actually reference like 'DRDs#name'.

@brutski Test to cover both cases around?