Resolve given variable

def resolve(self, _):
        """ Resolve given variable """
        if self.default_value == DUMMY_VALUE:
            if self.name in os.environ:
                return os.environ[self.name]
            else:
                raise VelException(f"Undefined environment variable: {self.name}")
        else:
            return os.environ.get(self.name, self.default_value)