Update codegen image

[weiqiangt]

Added openapi-gen installation for openapi spec generation.
Since the k8s.io/kube-openapi does not have tags, a workable commit hash is used.

[antrea-bot]

Thanks for your PR.
Unit tests and code linters are run automatically every time the PR is updated.
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[weiqiangt]

@antoninbas, could you please update the codegen image after merging this CL?

[weiqiangt]

/skip-all

[antoninbas]

LGTM

I pushed the new image to dockerhub already, no harm in doing so before this PR is merged.

What's the end goal of this PR - is this related to #323?

[weiqiangt]

LGTM

I pushed the new image to dockerhub already, no harm in doing so before this PR is merged.

What's the end goal of this PR - is this related to #323?

Yes, the openapi-gen will generate the openapi spec which fixes #323.