AWS_REGION should be optional

[willglynn]

Right now, the user has to specify WALE_S3_PREFIX and AWS_REGION. However, WALE_S3_PREFIX indicates an S3 bucket, S3 buckets have globally unique names, and the s3:GetBucketLocation API can take a S3 bucket name and return its region. AWS_REGION should therefore be optional, with wal-g able to determine the bucket's region automatically.

[fdr]

Unfortunately, it's not optional in every region, and new regions will make it less and less optional, on account of SigV4. GetBucketLocation is not always granted via IAM and I'm not too keen to expend extra API round-trips for this.

[willglynn]

I didn't say wal-g should disregard AWS_REGION, I suggested it should fall back to calling s3:GetBucketLocation if AWS_REGION were unspecified. Paired with #5, this would allow wal-g to run knowing only WALE_S3_PREFIX, with implicit defaults functioning for all the other parameters.

Would you accept a PR to this effect?