refactor: use $environment_type & $environment_name

If you disabled the GitLab-managed Terraform state (by setting `$TF_GITLAB_BACKEND_DISABLED` to `true`),

the template supports an implicit [backend configuration](https://www.terraform.io/language/settings/backends/configuration#file) mechanism:

1. Looks for a `$env.tfbackend` file (ex: `staging.tfbackend` for staging environment),

1. Looks for a `$environment_type.tfbackend` file (ex: `staging.tfbackend` for staging environment),

2. Fallbacks to `default.tfbackend` file.

If one of those files are found, it is automatically used by the template in the `terraform init` command (using the `-backend-config` CLI option).

1. Use [`tfvars` files](https://www.terraform.io/language/values/variables#variable-definitions-tfvars-files) for non-secret configuration:

    * default `terraform.tfvars[.json]` and `*.auto.tfvars[.json]` files are obviously supported by Terraform,

    * the template also auto-detects any file named `$env.env.tfvars[.json]` (ex: `staging.env.tfvars` for staging environment) and uses it with all related `terraform` commands.

2. any [predefined GitLab CI variable](https://docs.gitlab.com/ee/ci/variables/#predefined-environment-variables) may be freedly used in your hook scripts or extra options variables (ex: `TF_EXTRA_OPTS: "-var project_name=$CI_PROJECT_NAME"`)

    * the template also auto-detects any file named `$environment_type.env.tfvars[.json]` (ex: `staging.env.tfvars` for staging environment) and uses it with all related `terraform` commands.

2. any [predefined GitLab CI variable](https://docs.gitlab.com/ee/ci/variables/predefined_variables.html) may be freedly used in your hook scripts or extra options variables (ex: `TF_EXTRA_OPTS: "-var project_name=$CI_PROJECT_NAME"`)

3. you may also use [custom GitLab variables](https://docs.gitlab.com/ee/ci/variables/#custom-cicd-variables) to pass values to your hook script or even directly as Terraform variables [using the right syntax](https://www.terraform.io/docs/cli/config/environment-variables.html#tf_var_name)

    (ex: env variable `$TF_VAR_ssh_private_key_file` will be visible as `ssh_private_key_file` Terraform variable in your code)

4. **dynamic variables** provided by the template:

    log_info "--- \\e[32minit\\e[0m"

    log_info "--- Positioned environment (can also be used as TF vars):"

    # shellcheck disable=SC2154

    log_info "--- environment_type: \\e[33;1m${environment_type}\\e[0m"

    log_info "--- \$environment_type: \\e[33;1m${environment_type}\\e[0m"

    # shellcheck disable=SC2154

    log_info "--- environment_name: \\e[33;1m${environment_name}\\e[0m"

    log_info "--- \$environment_name: \\e[33;1m${environment_name}\\e[0m"

    # shellcheck disable=SC2154

    log_info "--- environment_slug: \\e[33;1m${environment_slug}\\e[0m"

    log_info "--- \$environment_slug: \\e[33;1m${environment_slug}\\e[0m"

    # maybe enable debug log

    if [[ "$TRACE" ]]; then

    # shellcheck disable=SC2154

    log_info "--- \\e[32mapply\\e[0m"

    # unset any upstream deployment env & artifacts

    rm -f terraform.env

    # maybe execute pre apply script

    prescript="$TF_SCRIPTS_DIR/tf-pre-apply.sh"

    if [[ -f "$prescript" ]]; then

    fi

    # finally propagate environment info

    # /!\ append is important here as it may have been created by a hook script

    echo -e "environment_type=$environment_type\\nenvironment_name=$CI_ENVIRONMENT_NAME\\nenvironment_slug=$CI_ENVIRONMENT_SLUG" >> terraform.env

  }