text: Title: Validate mandatory keys for API spec
  Body: ## Description
  
  We need to ensure that API spec has some mandatories keys in order to work properly.
  
  The first mandatory key that need to be checked is the key `api`. The specification should start with it.
  https://github.com/scanapi/scanapi/blob/master/scanapi/__init__.py#L68
  
  Under the key `endpoints`, we need to ensure each entry has **at least** a `name` and a `requests` key
  https://github.com/scanapi/scanapi/blob/master/scanapi/tree/endpoint_node.py#L79
  
  Under the key `requests`, we need to ensure each entry has **at least** a `name` and a `path` key.
  https://github.com/scanapi/scanapi/blob/master/scanapi/tree/request_node.py#L106
  
  This is an example of a minimal possible structure:
  
  ```yaml
  api:
    endpoints:
      - name: scanapi-demo
        requests:
          - name: health
            path: http://demo.scanapi.dev/api/health/
  ```
  
  If any of this mandatories keys is missing, an [error](https://github.com/scanapi/scanapi/blob/master/scanapi/errors.py) should be raised.