Description of problem:
null result is returned when orchestration template is deleted using REST API:
{
  "results": [
    null,
    null
  ]
}

For other collections (e.g. blueprints) meaningful result is returned:
{
  "results": [
    {
      "success": true,
      "message": "blueprints id: 3 deleting",
      "href": "https://<address>/api/blueprints/3"
    },
    {
      "success": true,
      "message": "blueprints id: 4 deleting",
      "href": "https://<address>/api/blueprints/4"
    }
  ]
}


Version-Release number of selected component (if applicable):
5.7.0.17


How reproducible:
100%


Steps to Reproduce:
1. create orchestration templates
2. delete orchestration templates:

POST /api/orchestration_templates
{
  "action" : "delete",
  "resources" : [
    { "href" : "https://<address>/api/orchestration_templates/:id1" },
    { "href" : "https://<address>/api/orchestration_templates/:id2" }
  ]
}