path based attributes/parameters aren't correctly validated

[Steven Hardy]

I noticed that invalid references to json parameters are allowed via path based parameters/attributes:

heat_template_version: ocata

parameters:
  RoleData:
     type: json
     default:
       step1: foo
       step3: foo

resources:

  value:
    type: OS::Heat::Value
    properties:
      value:
        step1: {get_param: [RoleData, step1]}
        step2: {get_param: [RoleData, step2]}
        step3: {get_param: [RoleData, step3]}
        step4: {get_param: [RoleData, step4]}
        anything: {get_param: [RoleData, noexist]}

This works, and creates a resource like:

| attributes | { |
| | "value": { |
| | "anything": "", |
| | "step4": "", |
| | "step3": "foo", |
| | "step2": "", |
| | "step1": "foo" |
| | } |
| | } |

We probably can't fix this without a template version bump, as I know some templates (I found this in the tripleo templates, which currently rely on this behavior...) may break if we fix it.

But I wanted to raise the issue as I think this is probably wrong and we should probably consider more correct validation in future.