Fix injection in OpenShift

[jwendell]

Fallback to single cluster client if multi cluster is not active.

[istio-policy-bot]

🤔 🐛 You appear to be fixing a bug in Go code, yet your PR doesn't include updates to any test files. Did you forget to add a test?

Courtesy of your friendly test nag.

[howardjohn]

when is multicluster not active?

[jwendell]

I keep getting unable to fetch namespace, failed to get client for ""

I'm guessing multicluster is false by default? https://github.com/istio/istio/blob/master/manifests/charts/istio-control/istio-discovery/values.yaml#L449

[jwendell]

I'm working on adding unit tests for this, but it might take a couple of days, can we merge this fix now and add UT later?

[howardjohn]

I keep getting unable to fetch namespace, failed to get client for ""

I'm guessing multicluster is false by default? https://github.com/istio/istio/blob/master/manifests/charts/istio-control/istio-discovery/values.yaml#L449

I feel like this should work. Maybe we need to make a lookup for "" to be the config cluster.

Either this issue impacts all of the multicluster components or none of them I would think; a fix localized to injection feels likely to be wrong/incomplete

[jwendell]

@howardjohn another approach: #49986

I'll add unit tests for openshift in a separate PR