[Network] fix SNAT remaping CIDR

[cheina97]

This PR fixes a huge bug we had in CIDR remapping. The SNAT rules for the pod-cidr and ext-cidr inside the gateway were populated with the wrong CIDRs.

Why wasn’t the bug spotted after a year and a half? Because when we perform remapping tests, we typically set the same pod CIDR and external CIDR on every cluster. In this particular case, the bug doesn’t occur because, in a similar scenario, the local pod CIDR and the remote CIDR (not the remapped one) are equal.

Should we consider changing the CIDR used in E2E tests to enhance entropy?

[adamjensenbot]

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[cheina97]

/rebase test=true

[fra98]

/merge