Add atmosphere_api logger to log most API failures. Update prepare_driver.

* Update prepare_driver to return None if it fails.
** Authorization fails.
** identity_id or provider_id are invalid.
* PEP8 Compliance.

	modified:   api/__init__.py
	modified:   atmosphere/settings/__init__.py
If prepare_driver returns None then fail with invalid_creds (401).

* Code needs to be in the actual method so it can return a response
  when there is an error. Given this model it can only be simplified
  so much.
* PEP8 Compliance.
* There are still some rare or special cases that should be handled.

	modified:   api/accounts.py
	modified:   api/application.py
	modified:   api/flow.py
	modified:   api/instance.py
	modified:   api/machine.py
	modified:   api/machine_export.py
	modified:   api/meta.py
	modified:   api/size.py
	modified:   api/volume.py
Merge branch 'atmo-273'