Remove DL0 specific code from custodiainstance in ipaserver/install
    
    iWithout DL0 support the custodia mode can be used to determine if a
    server or replica will be installed. Therefore the use of config.promote
    can be removed.
    
    A new check has been added to make sure the mode known in
    get_custodia_instance.
    
    See: https://pagure.io/freeipa/issue/7689
    Signed-off-by: Thomas Woerner <twoerner@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>