Add configurations to allow tempdir and Redshift cluster to be in different AWS regions

[JoshRosen]

By default, S3 <-> Redshift copies will not work if the S3 bucket and Redshift cluster are in different AWS regions. If you try to use a bucket in a different region, then you get a confusing error message; see https://forums.databricks.com/questions/1963/why-spark-redshift-can-not-write-s3-bucket.html for one example.

Note that it is technically possible to use a bucket in a different region if you pass an extra region parameter to the COPY command; see https://sqlhaven.wordpress.com/2014/09/07/common-errors-of-redshift-copy-command-and-how-to-solve-them-part-1/ for one example of this.

~~As a result, I think that we should document this limitation and possibly add some configuration validation to print a better error message when the S3 bucket is in the wrong region.~~~

We should add a configuration option so that users can explicitly specify the `tempdir` region to enable cross-region copies.

[JoshRosen]

Come to think of it, there are cases where we want to support cross-region transfers. Therefore, we might choose to split this into two separate issues: giving a more informative warning message and giving instructions on how to configure the cross-region UNLOAD command.

As far as I know, there's not an easy way to determine the cluster's region over JDBC, so I don't know that we'd be able to automatically figure out the correct UNLOAD command: http://stackoverflow.com/q/32545040/590203

[cfeduke]

Having aws_region as a configuration parameter might be enough. That's the first thing I looked for when I ran into this problem. (I created an S3 bucket in Standard to get around this problem.)

[JoshRosen]

That sounds reasonable to me; I was considering doing something similar. I think that tempdir_aws_region or s3_aws_region might be a slightly clearer configuration name, though.

[JoshRosen]

I'm a bit overloaded with other work at the moment and this task isn't part of our current sprint, so this is up-for-grabs if anyone wants to work on this. I do have time to review / revise small patches to spark-redshift if they won't involve too many rounds of back-and-forth.