Fix S3 destroy for feeds with sub paths

[emgarten]

• Updated AmazonS3FileSystem.GetFiles to account for sub paths when retrieving all files in the feed
  • This call is currently only used by Destroy, other scenarios should not be impacted
  • Previous behavior was resulting in the sub path being duplicated in the URI, which resulted in files not being found.
• Updated the S3 sub path functional test to verify the feed is fully removed.

Fixes https://github.com/emgarten/Sleet/issues/185