Drupal theme function for generating a bootstrap grid of nodes

example.module

/**
 * Implements hook_theme()
 */
function example_theme() {
  return array(
    'bootstrap_grid' => array(
      'render element' => 'elements'
    ),
  );
}

/**
* Generate a render array for bootstrap grid
* @param array $variables
* @return array $build (render array)
*/
function theme_bootstrap_grid($variables) {
  $css_class = 12 / $variables['bootstrap_columns'];
  unset($variables['nodes']['#sorted']);
  $rows = array_chunk($variables['nodes'],$variables['bootstrap_columns'],true);
  $build = array();

  foreach($rows as $id => $nodes) {

    $build[$id] = array(
      '#prefix' => '<div class="row">',
      '#suffix' => '</div>'
    );

    foreach($nodes as $nid => $node) {
      $build[$id][$nid] = array(
        '#prefix' => '<div class="col-md-'.$css_class.'">',
        '#suffix' => '</div>',
        'content' => $node
      );
    }
  }

  return $build;
}