code: def handle(cls, value, **kwargs):
          
          value = read_value_from_path(value)
  
          try:
              return os.environ[value]
          except KeyError:
              raise ValueError('EnvVar "{}" does not exist'.format(value))
docs: Retrieve an environment variable.
  
          For example:
  
              # In stacker we would reference the environment variable like this:
              conf_key: ${envvar ENV_VAR_NAME}
  
              You can optionally store the value in a file, ie:
  
              $ cat envvar_value.txt
              ENV_VAR_NAME
  
              and reference it within stacker (NOTE: the path should be relative
              to the stacker config file):
  
              conf_key: ${envvar file://envvar_value.txt}
  
              # Both of the above would resolve to
              conf_key: ENV_VALUE