Manifests-api: Added few tests related to manifests

Added the below tests:
* test_positive_create_1
* test_positive_delete_1
* test_negative_create_1
Similar comments apply.

```python
org_id = entities.Organization().create()['id']
...
task_id = entities.Organization(id=org_id).delete_manifest()
task_result = entities.ForemanTask(id=task_id).poll()['result']
```