Support configuring credentials via IAM roles and environment variables.

Problem/Motivation

The preferred way of passing key/secret credentials to the AWS SDK is via environment variables or IAM roles, rather than explicitly declaring the key/secret in configuration (e.g. $conf).
See http://docs.aws.amazon.com/aws-sdk-php/guide/latest/credentials.html

Proposed resolution

Allow checkbox for I am using environment variables or IAM roles to manage my key/secret.

Remaining tasks

Review

User interface changes

New checkbox - see screenshots.

Default - unchecked

Default - checked

API changes

None