Storwize driver cleanup

1. Moved Storwize driver into 'ibm' directory and refactored into
   separate files to improve readability and debug-ability. Other IBM
   drivers will be moved into this directory in a separate patch.
2. Errors coming from storage now raise VolumeBackendAPIException,
   and those coming from the driver raise VolumeDriverException.
3. Fixed many pylint errors.
4. Removed any usage of mox from unit tests in favor of mock.

DocImpact

Change-Id: I1696a97279efe834a96b3f3843bc65f0ea6da9cc
Merge "Storwize driver cleanup"