sssd_id.py: Primary group should be returned for initgroups
    
    Side effect of this change was that some primary groups
    could not be resolved and therefore get_user_groups
    failed in override tests. We should do the same as "id user".
    return decimal representation GID if it cannot be mapped to name.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit de19c0af27f1576c13bef183600136851baf767d)