rgw: Implementation for S3 Get Bucket Location

rgw: Implementation for S3 Get Bucket Location

S3 API supports getting the location for a bucket, which gives out one
of those geographic zones (US-WEST-1, EU for eg). Also it returns an
empty string for the default region.
(http://docs.aws.amazon.com/AmazonS3/latest/API/RESTBucketGETlocation.html)

Since LocationConstraint corresponds to regions in our case, this API
returns the region, for the "default" region empty string is returned

Signed-off-by: Abhishek Lekshmanan <abhishek.lekshmanan@ril.com>