use aws default credential provider if no access keys provided

[jaredstehler]

Uses the default credentials provider when no s3 access key or secret are provided in configuration:

AWS credentials provider chain that looks for credentials in this order:

1. Java System Properties - aws.accessKeyId and aws.secretAccessKey
2. Environment Variables - AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY
3. Web Identity Token credentials from system properties or environment variables
4. Credential profiles file at the default location (~/.aws/ credentials) shared by all AWS SDKs and the AWS CLI
5. Credentials delivered through the Amazon EC2 container service if AWS_CONTAINER_CREDENTIALS_RELATIVE_URI environment variable is set and security manager has permission to access the variable,
6. Instance profile credentials delivered through the Amazon EC2 metadata service

Adapted from #23