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's a question of contract. The contract, as read by me, is "pass a string to set, don't pass a parameter if you want system defaults. System defaults currently come from environment variables. In the future, we might add ~/.boto or ~/.aws or similar." 

The API happens to default to `aws_access_key_id=None` in the function call to detect if it was set. 

Meh. Not important. Changed. 

While we're at it, I also changed the default prefix to /xblock-storage/