playground: include '--comments' in mysql command line arguments

[mjonss]

What problem does this PR solve?

close #2186

What is changed and how it works?

It just adds --comments to the output to be used for running the mysql command line tool.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  ./bin/tidb-playground and verify output when cluster is ready.

Code changes

• output text change only

Side effects

• none

Related changes

• none

Release notes:

NONE

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• breezewish

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[breezewish]

Thanks!

[kaaaaaaang]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 908ab26