Adding the option to use an authenticated Docker user to pull the image

[zoltan-fedor]

Since Docker Hub's rate limit, you might need to pull the image with a DockerHub user.

What this PR does / why we need it:

Which issue this PR fixes

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Checklist

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• [ ] DCO signed
• [ ] Chart Version bumped
• [ ] Variables are documented in the README.md

[r-mironov]

Hello!

Im facing with the same problem when I need to add non default imagePullSecrets, looking forward to merging this PR

[zhilyaev]

I also need it

[zhilyaev]

@laurentderu, @mgoeminne , @Nico-Salamone Can you help us?