Update import tasks creation tests

* Removed import_from_format attribute passed for create import
* task smoke test since it is not mandatory also, removed its validation
* Added new method test_create_import_task_with_allowed_image_properties
* to validate create import task with allowed image properties

Change-Id: I8f54bfcf1bf798b0e6f1e6df7b8902cc719a59b8