[ntuple] fix set difference calculation in cluster preload
[ntuple] fix set difference calculation in cluster preload

karies/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,karies/root,olifre/root,olifre/root,karies/root,root-mirror/root,karies/root,olifre/root,karies/root,karies/root,olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root