Creating a new LocalStack session

[blairham]

Description

Issues related

• Assert failing if you are using local configuration for testing (i.e: localstack locally) #494
• No valid credential sources found for AWS Provider when running terratest in github actions #453

Pull Requests related

• Add aws custom configuration support for local testing (i.e: localstack use) #495

This adds a new create session that points to the URL specified in the env variable TERRATEST_LOCALSTACK. Since we don't need any credentials with LocalStack, we assume some random credentials and use the rest of Terratest. This was inspired by @ffernandezcast PR and the comments in it. This should be a much simpler approach and much more maintainable, which was one of the questions made by @brikis98

Usage

define the env var TERRATEST_LOCALSTACK

TERRATEST_LOCALSTACK=http://localhost:4566

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added support for custom AWS Endpoint.