conn: refine TLS configuration (#1575)

[ti-srebot]

cherry-pick #1575 to release-2.0
You can switch your code base to this Pull Request by using git-extras:

# In dm repo:
git pr https://github.com/pingcap/dm/pull/1578

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/dm.git pr/1578:release-2.0-5ebe8d9e5b7d

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What problem does this PR solve?

Fix CI for #1560. close #1560

What is changed and how it works?

Set TLS too even if we only have ssl-ca.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Related changes

• Need to cherry-pick to the release branch

[ti-chi-bot]

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