Warn when no region is configured

The AWS Java SDK V1 allows for cross region access. This means that even if you instantiate the S3 client with US_EAST_1 (or any region different to your actual bucket's region), the SDK will figure out the region. 

With the upgrade to SDK V2, this is no longer supported and the region should be set explicitly. Requests with the incorrect region will fail. To prepare for this change, S3A should warn when a region is not set via fs.s3a.endpoint.region. 

We should warn even if fs.s3a.endpoint is set and region can be parsed from this. This is because it is recommended to let the SDK V2 figure out the endpoint to use from the region, and so S3A should discourage from setting the endpoint unless absolutely required (eg for third party stores). 

Ideally rename fs.s3a.endpoint.region to fs.s3a.region, but not sure if this is ok to do.