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 
"it is there, and it is set to None. This is the same as not being there": Is that actually true? `djpyfs` treats both conditions the same (because it uses `None` as the default value if the setting is missing.)