Add wait for reading mycnf to prevent race

[GuptaManan100]

Description

This PR fixes the other issue pointed out in #14562 namely, there's an issue with mysqlctld creating my.cnf and vttablet racing to read it if both are started in parallel. That can result in the following error:

Error: mycnf read failed: open /vt/vtdataroot/vt_1320004125/my.cnf: no such file or directory

This wasn't a huge issue because it failed vttablets and when restarted, it would eventually work fine. But we can improve this experience by inherently having a wait time to read the cnf file if it isn't immediately available.

Related Issue(s)

• Fixes Bug Report: mysqlctld and vttablet startup is racy for a new instance #14562 along with the changes in Make vttablet wait for vt_dba user to be granted privileges #14565

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[dbussink]

Looks like this is already in the right spot so it only runs on managed MySQL instances.