Fixed different spell for region and default endpoint

Keep the same spell for all the region, add a test to replace us-east-1 to the default endpoint  s3.amazonaws.com.

It is fixing https://logstash.jira.com/browse/LOGSTASH-1319.
The pull request is a duplicate of https://github.com/logstash/logstash/pull/632 , but with the standard spell.