Manifests-api: Added few tests related to manifests

Added the below tests:
* test_positive_create_1
* test_positive_delete_1
* test_negative_create_1
To fix the below Travis issue you mentioned.

Both the functions `upload_manifest` and `delete_manifest` use `client._call_requests_*` under `Organizations` class in `entites` module.

My tests where failing when using client.post so,
I assumed it to be a **typo** and fixed it for `delete_manifest` function too.