Prevent Ref where Ref is not allowed

Ran into these today; making an issue here so they're not forgotten (I'd like to provide a patch+PR for this, but I can't take the time immediately).

- [ ] `GetAtt` will not validate if the parameters are not strings. I had `GetAtt(Ref(db_cluster), "Endpoint.Address")`, which worked fine on the troposphere side, but failed on the cfn side. Need `GetAtt(db_cluster, "Endpoint.Address")`
- [ ] similarly, `DependsOn` takes a string (or list of strings) and not a reference. `DependsOn=Ref(db_cluster)` fails on the cfn side; `DependsOn=db_cluster` works.