S3 blobstore region independent

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.

Regarding the tests, the problem is that Fog mock does not mimic the actual S3 behaviour, as described in [this Fog PR](https://github.com/fog/fog/pull/2461), so the issue goes undetected. What I did instead, is reuse all the existing S3 blobstore tests, using the European S3 endpoint. If you have suggestions on how to further address this issue, I would be happy to hear them!