Fix issue with S3 blobstore in non-standard endpoint

S3 returns a BucketAlreadyOwnedByYou error when attempting to create 
an already existing bucket in any region other than us-east-1. In us-east-1
the call is succesful. The fix resolves this issue that appears in the blobstore,
by first using get to fetch the bucket, and if that returns null, create the bucket.