Subusers cannot delete stacks with software config

[Drago]

Subusers which cannot create or delete Keystone users cannot delete stacks that have Nova servers with their user_data_format set to SOFTWARE_CONFIG even though no stack users are created. User deletion should be skipped if no user has been created (based upon the existence of a user_id in the resource's data [1]), but because a user ID is *always* returned [2], Heat always attempts to delete a user through Keystone. When this call to Keystone 403s, the stack deletion fails.

To reproduce:
- Create a subuser that cannot create users itself
- Create a stack using a template like [3]
- Delete the stack using the subuser
Expected behavior:
- Stack creates and deletes successfully
Actual behavior:
- Stack creates successfully
- Stack deletion results in DELETE_FAILED

[1] https://github.com/openstack/heat/blob/208d2debd0459ad5ce27884f8f4bbc3c1979ab18/heat/engine/resources/stack_user.py#L91-L93
[2] https://github.com/openstack/heat/blob/208d2debd0459ad5ce27884f8f4bbc3c1979ab18/heat/engine/resources/stack_user.py#L75-L83
[3]
heat_template_version: 2013-05-23

parameters:
  flavor:
    type: string
    description: Flavor for the server to be created
    default: m1.small
    constraints:
      - custom_constraint: nova.flavor
  image:
    type: string
    description: Image ID or image name to use for the server
    constraints:
      - custom_constraint: glance.image

resources:
  server:
    type: OS::Nova::Server
    properties:
      image: { get_param: image }
      flavor: { get_param: flavor }
      user_data_format: SOFTWARE_CONFIG
      user_data: { get_resource: config }

  config:
    type: OS::Heat::SoftwareConfig
    config: |
      #!/bin/bash
      echo foo

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/264351

[Sergey Kraynev (skraynev)]

Related patch was merged https://review.openstack.org/#/c/264351/