Cherry-pick: Check ARN for user ID strict check (#660)

[nckturner]

Cherry-pick of #660

What this PR does / why we need it:

Check ARN when mapping based on User ID. This is necessary because IAM User names/ARNs can change, and without this check, it would be difficult to determine which IAM users currently have access to a cluster.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nckturner

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[k8s-ci-robot]

@nckturner: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-aws-iam-authenticator-e2e	a5bfdbe	link	true	/test pull-aws-iam-authenticator-e2e

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[wongma7]

/lgtm