Fix configuration schema

Problem/Motivation

While trying to add tests for #2801993: CDN list is empty - Syntax error on CND JSON data, found config schema issues like below:

Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for bootstrap_library.settings with the following errors: bootstrap_library.settings:url missing schema, bootstrap_library.settings:theme missing schema, bootstrap_library.settings:minimized missing schema, bootstrap_library.settings:cdn missing schema, bootstrap_library.settings:files missing schema, bootstrap_library.settings:_core missing schema, bootstrap_library.settings:langcode missing schema in Drupal\Core\Config\Testing\ConfigSchemaChecker->onConfigSave() (line 93 of /www/htdocs/drupal8/core/lib/Drupal/Core/Config/Testing/ConfigSchemaChecker.php)

Proposed resolution

Fix it.