Add --cluster flag for filtering by cluster

[chancez]

This is just a short-cut to --node-name <cluster-name>/ but should make it more easier for users to filter by cluster, since many users are not aware the cluster name is part of the node name field on a flow.

[kaworu]

Patch LGTM, requesting change for a test func to be added for this new flag.

Also there is a potential incompatibility with the node-name flag, e.g.

hubble observe --cluster foo --node-name bar/cilium

Ideally we would check for this in the same fashion we do for e.g. the --from-namespace --from-pod since the "format" is the same (i.e. slash separated), but IMO non-blocking as --node-name is less useful than --{from,to}-pod and likewise for --cluster vs --{from,to}-namespace.

[glrf]

As is I think the following does something unexpected

hubble observe --cluster foo --node-name bar

I'd expect it to show me all flows related to node bar on cluster foo, but it'll show all flows on cluster foo as well as flows on node bar in any cluster.

So, I agree we should probably make --cluster incompatible with --node-name or implement the same logic I added for namespaces in #1279. But I think just making the flags incompatible should be good enough.

[chancez]

Hmm, good point. I had considered making them conflict, but found they seemed fine when using both flags to filter by cluster, but I didn't think about filtering by cluster + node, so I can see the possible confusing behavior. I'll update the PR.