'Returns a troposphere Ref to a value cached as a parameter.'

def cache_param(self, value):

    if (value not in self.cf_parameters):
        keyname = chr((ord('A') + len(self.cf_parameters)))
        param = self.cf_template.add_parameter(troposphere.Parameter(keyname, Type='String', Default=value))
        self.cf_parameters[value] = param
    return troposphere.Ref(self.cf_parameters[value])
