Support IPv6 in echoserver

[johngmyers]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This extends the echoserver so that it also listens on IPv6.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

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

[k8s-ci-robot]

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[dims]

@johngmyers tested this locally already?

[johngmyers]

Yes, tested a private amd64-only image build in an IPv6-only cluster.

[johngmyers]

curl http://'[fd00:5e4f:ce::e2b3]':80  


Hostname: echoserver-557bb6dc7c-tzhd4

Pod Information:
	-no pod information available-

Server values:
	server_version=nginx: 1.14.2 - lua: 10015

Request Information:
	client_address=2600:1f18:1e5f:8501:570d::6a35
	method=GET
	real path=/
	query=
	request_version=1.1
	request_scheme=http
	request_uri=http://[fd00:5e4f:ce::e2b3]:8080/

Request Headers:
	accept=*/*
	host=[fd00:5e4f:ce::e2b3]
	user-agent=curl/7.68.0

Request Body:
	-no body in request

[dims]

/approve

leaving lgtm for @mkumatag or other reviewers.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dims, johngmyers

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[oomichi]

/lgtm

[aojea]

I think that this functionality is already in the agnhost image 🤔