use testenv to separate conditions between production and testing
    
    - exclude secrets files only when the include_secrets grain is False
      (default is True)
    - adjust the prepare_test_env to include secrets files optionally (eg
      when running tests locally)
    - use 127.0.0.1 for ip4:interfaces:private when in testing, or real
      value when in production (in pillar/role/web_mirrors.sls for now, but
      it can be done in more places in the future