allow to custom private registry logic

[xuzhenglun]

What this PR does / why we need it:

related with #197

1. add the ability to custom private registry logic:
   for now, It always replaces the registry endpoint with a namespace (placeholder/ns/image_name --> private_registry/image_name ). But I want preserve namespace unchanged, (placeholder/ns/image_name --> private_registry/ns/image_name ). Compare with adding an new ImageRegistryTransform, leave applyPrivateRegistryToImage func customiable is a better way?

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Additional documentation:

[k8s-ci-robot]

Hi @xuzhenglun. Thanks for your PR.

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[atoato88]

/ok-to-test

[atoato88]

It looks good for me because it keep compatibility with current method and you can append additional logic for private repository manipuration.
I can't imagine that how much needs to change private repository we have, so I think current PR is OK.
/lgtm

[atoato88]

/assign @justinsb
Could you reveiw this?

[xuzhenglun]

ping @justinsb

@atoato88 or maybe we can ask someone else to review this? do you know anyone who is available?

[atoato88]

I added lgtm to this PR on Jan, and I approve now.
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: atoato88, xuzhenglun

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[atoato88]

@xuzhenglun
This PR have been merged, thank you to create this.
Could you add unit tests related to this PR as other PR(s) if possible?