fix bug for bucketname in file_exists() method on awsstorage.py
fix bug for bucketname in file_exists() method on awsstorage.py

ohaz/amos-ss15-proj1,ohaz/amos-ss15-proj1,ohaz/amos-ss15-proj1,ohaz/amos-ss15-proj1,ohaz/amos-ss15-proj1,ohaz/amos-ss15-proj1,ohaz/amos-ss15-proj1,ohaz/amos-ss15-proj1