AWS settings for XBlocks to have filesystem-like storage

Adds support for S3FS for deployment infrastructure. Tested on sandbox, and works. Waiting for tests. @cpennington @jarv 
Not sure what you mean by longer config files. It would make this settings file shorter (removing the `if` clause), and since it should use `AUTH_TOKENS.get('AWS_ACCESS_KEY_ID')` (note that `None` is the default default value for `.get`). It wouldn't force you to add values to `auth_tokens.json`, because it's still providing a default.