[ATMO-273] Consolidate to a single prepare_driver method.
	modified:   web/__init__.py
[ATMO-273] Use provider_id when creating driver.

* Add provider_id field to prepare_driver method.
* Enforce provider_id in django ORM to get Identity.
* PEP8 Compliance.

	modified:   api/__init__.py
	modified:   api/flow.py
	modified:   api/instance.py
	modified:   api/machine.py
	modified:   api/meta.py
	modified:   api/size.py
	modified:   api/step.py
	modified:   api/volume.py