Installer: activate nss and pam services in sssd.conf
    
    If there is already a sssd.conf file before the installer is
    executed, the nss and pam services may not be enabled by the
    installer. This happens for instance if the machine is hardened
    for STIG and sssd.conf does not define services=... in the
    [sssd] section.
    
    The consequence is that trust cannot be established with an AD
    domain.
    
    The installer must enable nss and pam services even if there is
    a pre-existing sssd.conf file.
    
    Fixes: https://pagure.io/freeipa/issue/9427
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>