Defer to default region provider chain rather than hardcoded us-east-1

[adrian-baker]

If no region is explicitly configured in com.gu.logback.appender.kinesis.BaseKinesisAppender#findRegion, then a hardcoded default of us-east-1 is used:

private Region findRegion() {
  boolean regionProvided = !Validator.isBlank(this.region);
  if(!regionProvided) {
    // Determine region from where application is running, or fall back to default region
    return Region.of(AppenderConstants.DEFAULT_REGION);
  }
  return Region.of(this.region);
}

It would be preferable to let the SDK default to the standard AWS region provider chain, which could be done simply by passing null into the client builder, by returning null, ie:

private Region findRegion() {
  return Validator.isBlank(this.region) ? null : Region.of(this.region); 
}