Allow the use of db credentials when verifying unmanaged tablets config

[frouioui]

Description

In planetscale/vitess-operator#671 we are starting to use the --unmanaged vttablet flag introduced in #14871. CI is showing an interesting failure where vttablet expects a password and user to be set via flags. The operator does set the user (code); but the password is instead stored in a K8S Secret object, and is passed down to vttablet via the --db-credentials-file flag.

However, with the current implementation, vttablet does not take into account the password stored in --db-credentials-file when verifying the unmanaged tablet settings and fail early since we have not set the --db_app_password flag.

This PR fixes this issue by making vttablet look at the credentials file before failing the unmanaged verification.

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.58%. Comparing base (7e24b08) to head (26ce556).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17984      +/-   ##
==========================================
- Coverage   67.59%   67.58%   -0.01%     
==========================================
  Files        1598     1598              
  Lines      259983   260050      +67     
==========================================
+ Hits       175727   175754      +27     
- Misses      84256    84296      +40     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.