ipatests: test third-party 389-ds cert with ipa-cert-fix
    
    ipa-cert-fix was hardcoded to use Server-Cert as the nickname
    so would fail if a third-party certificate was installed for DS.
    
    https://pagure.io/freeipa/issue/8600
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>