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 
Regarding #1: On sandbox machines, it is there, and it is set to None. This is the same as not being there. 

Regarding #2: I could do that if you prefer. As a matter of style, I prefer to omit irrelevant settings. They add clutter everywhere (inspecting variables when debugging, longer config files, etc.). In this particular case, the lines `"AWS_ACCESS_KEY_ID": null` get replicated among many config files (this is on sandbox, vagrant, etc.). 

Ideally, lines 213 and 214 would check command-line overrides, auth.json, environment variables, as well as possible some kind of platform-level defaults, so all settings at this level would be populated.