text: Title: Key(s) not found in format: <var_name> = '???' Error After upgrading to 1.0.
  Body: I have recently upgraded the tavern to 1.0 but after upgrading and changing all tests compatible to 1.0.
  Eg - ```body: -> json:``` in response.
  
  Almost tests are failing with format error.
  
  One scenerio:
  
  1. data.yaml
  
  ```yaml
  ---
  
  # Each file should have a name and description
  name: User Variables
  description: Varibales used for all tests for User
  
  # Variables should just be a mapping of key: value pairs
  variables:
    host: "{tavern.env_vars.TEST_API_HOST}"
    invalid_otp: 10000
    app_version: 50
    app_version_msg: "Please update to new version."
  ```
  2. test_user.tavern.yaml
  ```yaml
  includes:
    - !include data.yaml
  
  test_name: "User Initialization: environment setup (/api/v1/testing/integration/)"
  
  marks:
    - integration
  
  stages:
    - name: Initial data setup for accounts
      request:
          url: "{host}/api/v1/testing/integration/"
          method: POST
          json:
              setup: "accounts"
          headers:
              Content-Type: application/json
      response:
          status_code: 200
          json:
              success: true
              mobile: !anyint
              otp: !anystr
          save:
              json:
                  mobile: mobile
                  valid_otp: otp
  ---
  
  test_name: "Test User Login and Verify"
  
  marks:
    - integration
  
  stages:
    - name: Test for APP version check
      request:
          url: "{host}/api/v1/accounts/login/"
          method: POST
          json:
              mobile: "{mobile}"
      response:
          status_code: 400
          json:
              detail: "{app_version_msg}"
  ``` 
  In above test first test is passing, but second test is failing with below error. 
  
  ```': 'tests/accounts/test_user.tavern.yaml::Test User Login and Verify (call)'}}}]
  Key(s) not found in format: app_version_msg
    app_version_msg = '???'
  
  Source test stage (line 35):
    - name: Test for APP version check
      request:
          url: "{host}/api/v1/accounts/login/"
          method: POST
          json:
              mobile: "{mobile}"
      response:
          status_code: 400
          json:
              detail: "{app_version_msg}"
  
  
  Unable to get formatted stage
  
  Errors:
  E   tavern.util.exceptions.MissingFormatError: host
  ```
  
  And all my tests follow the same conventions, I save common variables in `data.yaml`, env_vars are loaded as well.
  
  I could not find any breaking changes regarding format in 1.0. Please address this if anyone know this error. 