Normalize internal implementation of 3 AWS resources

[clintoncwolfe]

Description

Three AWS resources, aws_iam_password_policy, aws_ec2_instance, and aws_iam_root_user were implemented early in the life of the project, using dependency injection for testing and 'resource' style AWS API calls. Those choices are valid in isolation, but all other AWS resources use a standardized approach using the AwsSingularResourceMixin or AwsPluralResourceMixin, which provides a variety of validation, testing, and (with #2613) exception handling facilities, allowing the resource code to be brief and distinct to the needs of the resource (and the AWS API that backs it).

Until the three resources are refactored, we must duplicate (or omit) any such shared functionality in the resources.

InSpec and Platform Version

InSpec 2.0 pre-release, branch release-2.0, 469369f