Get dbUser from Env or default('root')

[UrmsOne]

Fixes #984
After this PR,

• can dynamically configure the user name of mysql databases via environment variables
• can use external mysql instead of the mysql in pods

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This change is 

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[k8s-ci-robot]

Hi @UrmsOne. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[UrmsOne]

@googlebot I signed it!

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[gaocegege]

/ok-to-test

[hougangliu]

run gofmt -w ./pkg to pass test

[UrmsOne]

Thanks a lot!

[gaocegege]

We could remove this line in the future.

[hougangliu]

/lgtm

[gaocegege]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gaocegege

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[gaocegege]

@UrmsOne

Thanks for your contribution! 🎉 👍

Sorry for the late approval.