feat(glsh): add stgsub cluster to kube command

Background

In gitlab-com/gl-infra/infrastructure#14621 we are creating a new cluster to set up Prometheus inside of the cluster so that it can monitor the VMs.

How to set up

• At the moment the bastion server doesn't follow naming conventions, this is to be fixed in https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/merge_requests/3271 so you need to update your ~/.ssh/config with the following

  Host *.gitlab-subscriptions-staging.internal
    ProxyJump     lb-subscriptions-bastion.staging.gitlab.com

1. Run glsh kube setup
2. Run glsh kube use-cluster stgsub
3. Run kubectl get nodes and make sure they match https://console.cloud.google.com/kubernetes/clusters/details/us-east1/stgsub-customers-gke/nodes?project=gitlab-subscriptions-staging