Let deployer emit list of clusters for use with xargs

[consideRatio]

#3329 is about documenting how xargs can be used to do something once for each cluster. Currently though the strategy is to do ls config/clusters | xargs ... which makes a command be run "once per folder".

It would be nice to have a deployer command to replace the ls command, allowing us to also for example only get gke clusters or aks clusters etc.

I for example wanted to run a command once for each GKE cluster, but didn't want to be bugged by the EKS/AKS clusters that would return an error on the command i wanted to run for each cluster.

[yuvipanda]

Something like this exists now I believe.