A good enhancement in Drupal 8 is having list of yml file to import via installation.(in mymodule_root\config\install) 
the problem is I can not reinstall mymodule normally as configs already exist in active configuration.
Actually the problem is during uninstallation the imported configs imported in installation phase has not been waived in uninstall phase.

![enter image description here][1]

Test senario:
put this sample yml file in yourmodule_root\config\install\systme.site.yml

    name: 'Admissions at Adelaide'
    mail: [email protected]
    slogan: ''
    page:
      403: ''
      404: ''
      front: caslogin
    admin_compact_mode: false
    weight_select_max: 100
    langcode: en
    default_langcode: en



  [1]: https://i.sstatic.net/sTnA0.png