Update default part size other than 32MB

[Rishika Kedia]

Hello,

Is there a way to update default part size, during the MPU, other than 32MB for 
AWS-S3 blobstore for jclouds2.0? earlier for 1.9* version this was not issue as 
MultipartUploadSlicingAlgorithm.java was not made final class.

Thanks,
Rishika