Fix internal ingress static IP bug

[spencerhance]

We mistakenly provided the cluster subnet for the forwarding rule, which prevents customers
from being able to use a static ip from any subnet in the same VPC/Region as the cluster

[k8s-ci-robot]

Hi @spencerhance. Thanks for your PR.

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[spencerhance]

/assign @freehan

Minhan, I'm assigning to you since you reviewed the original code. Feel free to reassign.

[bowei]

/ok-to-test

[freehan]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: freehan, spencerhance

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[spencerhance]

/retest