Returns the function object that matches the provided name.
        Only Fn:: and Ref functions are supported here so that other
        functions specific to troposphere are skipped.

def _get_function_type(self, function_name):
        """
        Returns the function object that matches the provided name.
        Only Fn:: and Ref functions are supported here so that other
        functions specific to troposphere are skipped.
        """
        if (function_name.startswith("Fn::") and
                function_name[4:] in self.inspect_functions):
            return self.inspect_functions[function_name[4:]]
        return (self.inspect_functions['Ref'] if function_name == "Ref"
                else None)