Set a runner as idle: - runner_id - String, unique identifier for runner - callback - f(err)
function idleRunner(runner_id, callback) {
            if (!runners[runner_id]) {
                return callback(new e.BackendResourceNotFoundError(sprintf(
                  'Runner with uuid \'%s\' does not exist', runner_id)));
            } else {
                runners[runner_id].idle = true;
                return callback(null);
            }
        }