Add a check in ConfirmUsable() to validate the contextName

[asalkeld]

What this PR does / why we need it:
When a context name is provided, but can't be found (miss spelling), it currently
uses the defaults. This PR will cause the command to fail, to prevent unexpected side effects
of using the wrong configuration.

Which issue this PR fixes
fixes #21062

Special notes for your reviewer:
None

Release note:

Error if a contextName is provided but not found in the kubeconfig.

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This change is

[k8s-bot]

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[k8s-bot]

Can one of the admins verify that this patch is reasonable to test? If so, please reply "ok to test".
(Note: "add to whitelist" is no longer supported. Please update configurations in kubernetes/test-infra/jenkins/job-configs/kubernetes-jenkins-pull instead.)

This message will repeat several times in short succession due to jenkinsci/ghprb-plugin#292. Sorry.

[k8s-bot]

GCE e2e build/test passed for commit e9cad12.

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[k8s-github-robot]

@k8s-bot test this [submit-queue is verifying that this PR is safe to merge]

[k8s-bot]

GCE e2e build/test passed for commit e9cad12.

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[k8s-github-robot]

Automatic merge from submit-queue