Testing updates

[troyready]

A few testing updates:

• Most notably, there's an AWS cleanup bug that can sometimes cause the terraform apply/destroy cycle to fail (this isn't specific to the automated setup or running on CI). Now the tests will watch for those lingering ENIs & Security Groups and purge them during the destroy process.
• When running locally (not in CI), the test script will now ensure that the Terraform workspace name is persisted to disk. This should be useful in debugging sessions -- now one can just switch to the relevant module directory and run terraform commands without selecting the workspace again.
• Finally, dropped legacy terraform-credentials-env plugin use now that tests only use TF 1.2+