Added nullstone iac test command

[BSick7]

This PR adds the nullstone iac test command which allows a developer to verify their IaC file before pushing and running through a gitops workflow.

Example:

nullstone iac test --stack=core --env=prod

This reports validation errors with the IaC files and provides a high-level summary changes that will happen to the specified environment.

Specifically, this command:

1. Discovers IaC files in a repo (based on the executing directory)
2. Processes the IaC files (parse, resolve, validate, normalize)
3. Applies changes to a local copy of the workspace config