Remove unused exceptions

The following exceptions are not used.
  class MigrationNotFound(NotFound):
  class BadDriverResponseStatus(VolumeDriverException):
  class InstanceNotFound(NotFound):
  class VolumeRetypeFailed(CinderException):
  class DuplicateSolidFireVolumeNames(SolidFireDriverException):

Change-Id: I891360f9c150eb8ea3f3968f12b9ad3da998981a
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
GlusterFS: Fix deadlock in volume clone

The create_cloned_volume path could deadlock due to
create_cloned_volume and create/delete_snapshot using the same
lock for synchronization.

Refactor the calls to create/delete snapshot to call the inner
method which does not use a lock.

Introduced by "06999f6 GlusterFS: Synchronize additional op..."

Related-Bug: 1267983
Closes-Bug: 1272092

Change-Id: I84ca34b201c10644faa047f1c9274c14bcdd0359
Redundant check in os-migrate_volume_completion

Remove the redundant check for 'migrate_volume_completion', and add the UT
for it.

Change-Id: I6b643dbf9ae1302cafe6748a361123d7d76c437f
Closes-Bug: #1254963
Merge "Storwize driver cleanup"
Merge "Redundant check in os-migrate_volume_completion"
Merge "GlusterFS: Fix deadlock in volume clone"
Merge "Remove unused exceptions"