retrieves the config variable named ``path`` from the config file.
        Path elements that start with a $ are replaced with values from the
        dict passed as ``env``

def get_config_var(self, path, env=None):
        '''
        retrieves the config variable named ``path`` from the config file.
        Path elements that start with a $ are replaced with values from the
        dict passed as ``env``
        '''
        if env is None:
            env = {}
        result = self.conf
        for e in path.split('.'):
            if e[0] == '$':
                try:
                    result = result[env[e[1:]]]
                except KeyError as ex:
                    try:
                        result = result['.default']
                    except KeyError:
                        raise ex
            else:
                result = result[e]
        return result