Add Terraform Cloud backend. Add/update docs

[aknysh]

what

• Add Terraform Cloud backend
• Add docs:
  • Terraform Workspaces in Atmos
  • Terraform Backends. Describes how to configure backends for AWS S3, Azure Blob Storage, Google Cloud Storage, and Terraform Cloud

why

• Allow configuring and using the Terraform Cloud backend
• Document existing and new Atmos features

docs

Terraform Cloud backend uses a cloud block to specify which organization and workspace(s) to use.

To configure the Terraform Cloud backend in Atmos, add the following config to an Atmos manifest in _defaults.yaml:

terraform:
  backend_type: cloud
  backend:
    cloud:
      organization: "my-org"
      hostname: "app.terraform.io"
      workspaces:
        # Parameters for workspaces

For each component, you can optionally specify the workspaces.name parameter similar to the following:

components:
  terraform:
    my-component:
      # Optional backend configuration for the component
      backend:
        cloud:
          workspaces:
            name: "my-component-workspace"

If auto_generate_backend_file is set to true in the atmos.yaml CLI config file in the components.terraform section,
Atmos will deep-merge the backend configurations from the _defaults.yaml manifests and from the component itself, and will generate a backend config JSON file backend.tf.json in the component's folder, similar to the following example:

{
  "terraform": {
    "cloud": {
      "hostname": "app.terraform.io",
      "organization": "my-org",
      "workspaces": {
        "name": "my-component-workspace"
      }
    }
  }
}

Instead of specifying the workspaces.name parameter for each component in the component manifests, you can use
the {terraform_workspace} token in the cloud backend config in the _defaults.yaml manifest.
The token {terraform_workspace} will be automatically replaced by Atmos with the Terraform workspace for each component. This will make the entire configuration DRY.

terraform:
  backend_type: cloud
  backend:
    cloud:
      organization: "my-org"
      hostname: "app.terraform.io"
      workspaces:
        # The token `{terraform_workspace}` will be automatically replaced with the
        # Terraform workspace for each Atmos component
        name: "{terraform_workspace}"