code: def substitute_vars(template, replacements):
      
      result = template
      for (key, value) in replacements:
          result = result.replace('${' + key + '}' , value)
      if '${' in result:
          logging.warning("A variable was not replaced in '%s'.", result)
      return result
docs: Replace certain keys with respective values in a string.
      @param template: the string in which replacements should be made
      @param replacements: a dict or a list of pairs of keys and values