Add support for pass through references

To make template resources more transparent we now support
the user adding an output "OS::stack_id" to a nested stack. This
is then used by get_resource in the outer stack.

Note: it is expected to use this as follows:
outputs:
  OS::stack_id:
    value: {get_resource: my_server}

bp env-nested-usability
Change-Id: I6016699514e75bd88c27f9f8f7d871b39d31343d
Merge "Add support for pass through references"