Ensure config entity id is set for computed composite ids

There is some duplicate code in a couple of places related with computed ids for config entities:

  /**
   * {@inheritdoc}
   */
  public function save() {
    // Build an ID if none is set.
    if (empty($this->id)) {
      $this->id = $this->id();
    }
    return parent::save();
  }

This should be moved to ConfigStorageController::create() (if the id is accessible / not protected) or ConfigEntityBase::preSave

Related: #2020895: Move save() / delete() logic in Field / FieldInstance to [pre|post]Save(), [pre|post]Delete()