Dont set -num-nodes on karpenter-managed clusters

[rifelpet]

Fixes a panic during karpenter e2e tests introduced by #16176

https://prow.k8s.io/view/gs/kubernetes-jenkins/logs/e2e-kops-aws-karpenter/1754176571796623360

I0204 16:34:13.468238   14042 tester.go:75] Adding --host=https://api-e2e-e2e-kops-aws-karp-3r2ait-3bb67354234fd44a.elb.eu-central-1.amazonaws.com/
I0204 16:34:14.085494   14042 tester.go:158] Setting --provider=aws
I0204 16:34:14.808979   14042 tester.go:177] Setting --gce-zone=eu-central-1a
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x2e280fd]
goroutine 1 [running]:
k8s.io/kops/tests/e2e/pkg/tester.(*Tester).addNodeIG(0xc000c6f260)
	/home/prow/go/src/k8s.io/kops/tests/e2e/pkg/tester/tester.go:202 +0xbd
k8s.io/kops/tests/e2e/pkg/tester.(*Tester).execute(0xc000c6f260)
	/home/prow/go/src/k8s.io/kops/tests/e2e/pkg/tester/tester.go:465 +0x193
k8s.io/kops/tests/e2e/pkg/tester.Main()
	/home/prow/go/src/k8s.io/kops/tests/e2e/pkg/tester/tester.go:512 +0xc5
main.main()
	/home/prow/go/src/k8s.io/kops/tests/e2e/kubetest2-tester-kops/main.go:24 +0xf

Example IG manifest

/cc @upodroid

[upodroid]

Thanks

[hakman]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hakman, upodroid

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [hakman]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment