cluster: auto session certs

[xhebox]

What problem does this PR solve?

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• [x] Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

NONE

[ti-chi-bot[bot]]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 63.10680% with 38 lines in your changes are missing coverage. Please review.

Project coverage is 55.67%. Comparing base (79c584f) to head (d382d73). Report is 1 commits behind head on master.

:exclamation: Current head d382d73 differs from pull request most recent head ecff4e7. Consider uploading reports for the commit ecff4e7 to get more accurate results

Files	Patch %	Lines
pkg/cluster/manager/builder.go	63.24%	17 Missing and 8 partials :warning:
pkg/cluster/spec/tidb.go	56.67%	11 Missing and 2 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2374      +/-   ##
==========================================
- Coverage   55.78%   55.67%   -0.11%     
==========================================
  Files         329      329              
  Lines       35024    35100      +76     
==========================================
+ Hits        19537    19541       +4     
- Misses      13143    13208      +65     
- Partials     2344     2351       +7     

Flag	Coverage Δ
cluster	45.19% <62.00%> (-0.11%)	:arrow_down:
dm	25.47% <8.00%> (-0.04%)	:arrow_down:
playground	15.30% <8.57%> (-0.03%)	:arrow_down:
tiup	33.60% <ø> (ø)
unittest	22.19% <0.00%> (-0.05%)	:arrow_down:

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