CONFIG: Add a new option auto_private_groups
    
    The auto_private_groups option is used to configure the domain->mpg flag
    which was already set automatically for subdomains, but for some time was
    not settable by the admin via the configuration file.
    
    The new option name, instead of the old magic_private_groups, was chosen
    purely because this name would hopefully be better understood by admins.
    
    The option doesn't do anything yet, it is just added to all the places a
    new option should be added to.
    
    Related:
        https://pagure.io/SSSD/sssd/issue/1872
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>