Use configuration file or default values for user SSH keys if not provided in arguments (#7)

src/main/python/stratuslab/libcloud/compute_driver.py: In create_node function, if auth is argument is None or not provided, or not specified in configuration file, use default 'userPublicKeyFile' value, override deploy_node function so as to use configuration file or default 'userPrivateKeyFile' value, if ssh_key argument is not provided