add IPv6 listen directive to nginx if enable_dual_stack_networks

[kakkotetsu]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

In a dual stack cluster with kubelet --node-ip <IPv6 addr>,<IPv4 addr>, IPv6 is preferred. In that case, the IP of the nginx-proxy pod will also be IPv6, so IPv6 listen directive is required. This PR add it.
(This is not required, since kubespray is set to kubelet --node-ip <IPv4 addr>,<IPv6addr> at the moment.)

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Add IPv6 listen directive to nginx if enable_dual_stack_networks

[k8s-ci-robot]

Hi @kakkotetsu. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[cristicalin]

Thanks for this @kakkotetsu !

/ok-to-test
/lgtm

[floryut]

@kakkotetsu 🍾

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, kakkotetsu

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