Fixing catalog listing/instantating from REST via yaml

@sjcorbett @neykov can you review please.

I'm still investigating + fixing why running all the tests in a row fails.
i don't think there's (yet) a compelling use case for a catalog having multiple top-level entities.  i can see how it might be useful but could be a bit tricky.

suggest removing TODO here.  (could keep rest of comment "in case we want to ....")