Fix busybox version in integration tests

[u5surf]

fix #4475
To resolve domain name with nslookup , we should use busybox:1.28.4 instead of latest in the integration tests.
c.f. docker-library/busybox#48

[k8s-ci-robot]

Hi @u5surf. Thanks for your PR.

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[k8s-ci-robot]

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[minikube-bot]

Can one of the admins verify this patch?

[u5surf]

travis failed the reason why it seems to be errored to install linter.

= make lint =============================================================
go: downloading github.com/golangci/golangci-lint v1.16.1-0.20190425135923-692dacb773b7
go: extracting github.com/golangci/golangci-lint v1.16.1-0.20190425135923-692dacb773b7
go: github.com/Quasilyte/go-consistent@v0.0.0-20190521200055-c6f3937de18c: parsing go.mod: unexpected module path "github.com/quasilyte/go-consistent"
go get: error loading module requirements
golangci/golangci-lint info checking GitHub for tag 'v1.16.0'
golangci/golangci-lint info found version: 1.16.0 for v1.16.0/linux/amd64
make[1]: *** [out/linters/golangci-lint] Error 1
= boilerplate 

I think the linter is collapsed the dependency of the modules following this issue
quasilyte/go-consistent#34

It is hard to think it has occurred due to change the pod image’s version

[medyagh]

@minikube-bot okay to test

[medyagh]

Thank for this PR, if this works it is actually a great help ! I restarted the build test...

[medyagh]

could you please change it across all other busbox pods ?

for example this one ? minikube/test/integration/testdata/busybox-mount-test.yaml

[medyagh]

@minikube-bot okay to test

[medyagh]

@minikube-bot OK to test

[medyagh]

the testVersionUpgrade failure is related to #4418

[medyagh]

Thank you so much @u5surf for the great fix, this literally saved me hours of investigating ! great contribution.