Update to use kubernetes 1.15 as dependency

[leakingtapan]

Is this a bug fix or adding new feature?
Since k8s 1.15+ uses go mod for vendering dependencies, it causes problem for dependencies using replace directive.

Add update-gomod.sh to update go.mod when consuming k/k as go module

Ref: kubernetes/kubernetes#79384

[k8s-ci-robot]

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This pull-request has been approved by: leakingtapan

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[coveralls]

Pull Request Test Coverage Report for Build 755

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.1%

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Totals
Change from base Build 751:	0.0%
Covered Lines:	1176
Relevant Lines:	1654

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💛 - Coveralls

[wongma7]

Nice! TBH I kind of hate that we have to do this, I almost want to stay with dep, but dep is super slow. Anyway it's k/k's fault. Will go mod commands like tidy and vendor still work?

[wongma7]

/lgtm

[leakingtapan]

Will go mod commands like tidy and vendor still work?

go mod vendor still works that's how I get to test this change locally. go mod tidy seems to still works. But I haven't tested it enough

[wongma7]

/retest