Add AWS Region support to PhutilAWSFuture.

Summary:
Almost [[http://docs.aws.amazon.com/general/latest/gr/rande.html|all]] AWS services endpoints follow this structure: `SERVICE_NAME.REGION_ID.amazonaws.com`. This revision adds support for setting a region in `PhutilAWSFuture`.

There are couple of exceptions to above rule - some services do not have region in their endpoint url (DevPay, Route 53, IAM, Import/Export, Security Token Service), some have a single endpoint in one region (OpsWorks, Support), some are //funky// (S3), but they all can be covered by overriding `getHost()` method.

Test Plan: No tests added. I `ack`ed through three repositories (`libphutil`, `arcanist` & `phabricator`) and couldn't find AWS used anywhere.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8311