feat(ecs): allow to specify log retention for aws log driver

[jogold]

Allow to specify a log retention when using the AWS Log Driver with an automatically created group.

Also add tests for the driver.

Note: this can of course already be achieved today by first creating the log group and then passing it to the driver, but I think it's a better user experience to allow to specify the retention directly.

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Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

[jogold]

TO BE DISCUSSED: What should be the default? Never expire like in Lambda (this PR's proposal), 365 like it is right now, 731 like the default for a new log group? This needs uniformity...