Change ckeditor table dialogue defaults
You can achieve this in several ways with a script we can call, for example, ckeditor_settings_modifier.js containing: 

CKEDITOR.on('dialogDefinition', function(ev) {
  var dialogName = ev.data.name;
  var dialogDefinition = ev.data.definition;

  if(dialogName == 'table') {
    var info = dialogDefinition.getContents('info');
    info.get('txtWidth')['default'] = '100%';
  }
});


This script should be added in one of these places through libraries:


only in the theme of your website (if you use it also for editing)
in your administration theme in order it works in normal node edit mode, and in your website theme in order it works in quick edit mode
in a custom module in which you can use for example some conditional rules that control the loading of the script. 


In any choosen settings it is important to declare the dependency on the core/ckeditor. 

This is the case of a module (my_module containing a js folder with ckeditor_settings_modifier.js), in my_module.libraries.yml: 

ckeditor_settings_modifier:
  version: 1.x
  js:
    js/ckeditor_settings_modifier.js: {}
  dependencies:
    - core/ckeditor


in my_module.module: 

/**
 * Implements hook_preprocess_page().
 */
function my_module_preprocess_page(&amp;$variables) {
  // add ckeditor_modifier in case the user has the role of administrator
  $roles = Drupal::currentUser()-&gt;getRoles();
  if(in_array('administrator', $roles)) {
    $variables['#attached']['library'][] = 'my_module/ckeditor_settings_modifier';
  }
}


It works on Drupal 8.4.4 and CKEditor 4.8