clush: add -i for nodes intersection

[thiell]

During internal ClusterShell seminar, someone brought up the fact that clush doesn't have a -i option for nodeset intersection (like nodeset CLI). Maybe this should be added.

NOTES

Due to pdsh group -g/-X compat options, we cannot add --xor/-X operation in clush for now... but this is less used, and extended patterns (eg. -w nodeset1^nodeset2) can still be easily used.