fix: skip instance not found when decoupling vmss from lb

[nilo19]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #787

Special notes for your reviewer:

Does this PR introduce a user-facing change?

fix: skip instance not found when decoupling vmss from lb

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nilo19

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[coveralls]

Coverage decreased (-0.02%) to 80.308% when pulling 0b7ba30 on nilo19:fix/skip-not-found into 6bdd6e2 on kubernetes-sigs:master.

[feiskyer]

/lgtm

Let's also cherrypick to old releases and k/k

[nilo19]

/retest

[MartinForReal]

/cherrypick release-1.1

[MartinForReal]

/cherrypick release-1.0

[k8s-infra-cherrypick-robot]

@MartinForReal: new pull request created: #1433

In response to this:

/cherrypick release-1.1

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[k8s-infra-cherrypick-robot]

@MartinForReal: new pull request created: #1434

In response to this:

/cherrypick release-1.0

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.