orchestration: add test for untyped provider resources

It's also possible to specify the filename directly in the template
without adding the type alias to resource_registry, then heat looks
up the provider/nested-stack resource in the "files" map passed when
creating the stack.

Change-Id: Idce23021dd4ddcedef61ffdd39d6abb16885e1be
Partial-Bug: #1317095
Merge "orchestration: add test for untyped provider resources"