Portworx: Parse the storage class mapping while restoring a volume.
Portworx: Parse the storage class mapping while restoring a volume.

This change allows backing up a Portworx volume of a particular storage class
and properties, and restore that volume with a different storage class.
For ex. Backup a volume with HALevel=2 and restore it with HALevel=1

- Parse the storage class mapping from restore spec and find if a mapping
  for the current PVC's storage class is found.
- If found, fetch the actual contents of storage class and parse them into
  a RestoreVolumeSpec.
- Invoke the CloudBackupRestore API with this RestoreVolumeSpec.

libopenstorage/stork,libopenstorage/stork,libopenstorage/stork