Get the given runner id details - runner_id - String, unique identifier for runner. Required. - callback - f(err, runner)
function getRunner(runner_id, callback) {
            if (!runners[runner_id]) {
                return callback(new e.BackendResourceNotFoundError(sprintf(
                  'Runner with uuid \'%s\' does not exist', runner_id)));
            } else {
                return callback(null, runners[runner_id].active_at);
            }
        }