[users] Move default groups setting to Config

- drop groups from the viewstep
- note that the Config object should also be in charge of creating
  Jobs (but then the de-tangling needs to be completed)
- add tests of default groups loading

Doesn't compile because QRegExpValidator is a gui thing.

src/modules/users/UsersViewStep.h

@@ -61,7 +61,6 @@ private:
     UsersPage* m_widget;
     QList< Calamares::job_ptr > m_jobs;
 
-    QStringList m_defaultGroups;
     SetHostNameJob::Actions m_actions;
 
     Config* m_config;