Grab credentials from config files

[clifflu]

Try to grab credentials from config files if not supplied from Credentials::factory() or Options. Supported formats include aws-cli, aws-credential (old awscli), and boto, detected in alphabetical order.

Test cases included, also fixed the issue to fail some tests if env AWS_ACCESS_KEY_ID and AWS_SECRET_KEY were set.

[mtdowling]

Thanks for the PR. We're interested in being able to use the AWS CLI configuration format, but we're still trying to determine to what extent we will support the format.

We're less interested in being able to use the old config format or the Boto config files due to the additional service area, complexity, and implied commitment to supporting these formats now and in the future (i.e., if they change the format). Would you mind removing everything except for the AWS CLI configuration parsing? After that's done, we can more thoroughly review your code.

[clifflu]

No problem. Should be done this weekend :)

[skyzyx]

👍 for supporting AWS CLI config syntax.

[mtdowling]

Just want to give you a heads up that we'll likely support the same credentials file as the CLI, but the credentials file that is shared by the CLI and the SDK may change. When integrating this, we'd likely need to make other significant changes (like possibly supporting something equivalent to the CLI's "profiles"). Because this is a larger scoped issued that needs to be resolved first internally, I'm going to close this PR.