EC2, Enable authentication using default credential chain

[tluimes]

Signed-off-by: Tim Luimes [email protected]

Enables the ability for the ec2 client session to be authenticated using methods provided by the aws SDK's default credential chain when the accessKey and secretKey aren't provided.

https://docs.aws.amazon.com/sdk-for-go/v1/developer-guide/configuring-sdk.html
"When you initialize a new service client without providing any credential arguments, the SDK uses the default credential provider chain to find AWS credentials. The SDK uses the first provider in the chain that returns credentials without an error. The default provider chain looks for credentials in the following order:

Environment variables.

Shared credentials file.

If your application uses an ECS task definition or RunTask API operation, IAM role for tasks.

If your application is running on an Amazon EC2 instance, IAM role for Amazon EC2."

[derek]

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