Always set instance region but force SigV4 authentication when trying to get bucket location

[zhan849]

S3 boto3 client needs to initialize with bucket's region if the region is using signature V4. Currently we are getting region as follows:

1. Try HeadBucket, but ignore error, as region name is included in header. If does not work,
2. Try GetBucketLocation, find region, if does not work,
3. Use HTTP version of virtual host URL <bucket-name>.s3.amazonaws.com according to discussion under GetBucketLocation fails if region specified does not match the region the bucket is in. aws/aws-sdk-go#720 (comment) as a last resort

This solution works perfectly fine in non-GovCloud partition, but does not work in GovCloud as reported by people in community @lorengordon.

A better solution would be (assume cluster don't access resources across partition, which is a valid one):

1. Use AWS metadata to get instance region
2. Instantiate a boto3 Session object with that region, and then a S3 client but force sigv4 auth (According to http://docs.aws.amazon.com/general/latest/gr/signing_aws_api_requests.html, all regions support signature V4)

s3 = boto3.Session(profile_name=<prof>, region_name=<reg>).client('s3', config=Config(signature_version='s3v4'))

3. Use head bucket to get bucket region
4. Instantiate actual bucket client use actual region