WIP: upgrade openapi generator to v5.1.1

[roycaihw]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR builds on top of #1478.

Upgrade openapi-generator to v5.1.1, to include upstream new features and bug fixes.

Note that the python generator is used, which contains a huge refactoring and uses python3 only: OpenAPITools/openapi-generator#7965

Which issue(s) this PR fixes:

Fixes #1421

Does this PR introduce a user-facing change?

[k8s-ci-robot]

@roycaihw: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: roycaihw

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[k8s-ci-robot]

@roycaihw: PR needs rebase.

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/lifecycle stale

[roycaihw]

/close

Will give it another try in latest master

[k8s-ci-robot]

@roycaihw: Closed this PR.

In response to this:

/close

Will give it another try in latest master

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