cluster: upgrade PD leader after other PD instances

[AstroProfundis]

What problem does this PR solve?

Close #664

What is changed and how it works?

Check if instance is PD leader and defer the upgrading process for it.

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change

Side effects

• Increased code complexity

Release notes:

cluster: upgrade PD leader node after upgrading other PD nodes

[codecov-io]

Codecov Report

Merging #1086 (2ade913) into master (d4881ba) will decrease coverage by 4.53%.
The diff coverage is 65.90%.

@@            Coverage Diff             @@
##           master    #1086      +/-   ##
==========================================
- Coverage   56.41%   51.87%   -4.54%     
==========================================
  Files         278      278              
  Lines       19696    19723      +27     
==========================================
- Hits        11111    10232     -879     
- Misses       6862     7870    +1008     
+ Partials     1723     1621     -102     

Flag	Coverage Δ
cluster	38.46% <65.90%> (-6.17%)	⬇️
dm	24.34% <22.72%> (-0.02%)	⬇️
integrate	46.15% <65.90%> (-4.61%)	⬇️
playground	20.91% <ø> (ø)
tiup	16.67% <0.00%> (-0.02%)	⬇️
unittest	22.40% <0.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/cluster/spec/pd.go	68.07% <61.53%> (-0.72%)	⬇️
pkg/cluster/operation/upgrade.go	68.88% <67.74%> (-1.49%)	⬇️
components/cluster/command/check.go	6.60% <0.00%> (-72.96%)	⬇️
pkg/cluster/task/limits.go	0.00% <0.00%> (-68.75%)	⬇️
pkg/cluster/task/sysctl.go	0.00% <0.00%> (-66.67%)	⬇️
pkg/cluster/task/copy_file.go	0.00% <0.00%> (-54.55%)	⬇️
components/cluster/command/audit.go	27.27% <0.00%> (-54.55%)	⬇️
pkg/cluster/operation/check.go	0.00% <0.00%> (-53.07%)	⬇️
pkg/cluster/manager/patch.go	0.00% <0.00%> (-51.81%)	⬇️
pkg/cluster/task/rmdir.go	0.00% <0.00%> (-50.00%)	⬇️
... and 41 more

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[lucklove]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• lucklove

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[lucklove]

/merge

[ti-chi-bot]

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

Commit hash: 1574ec7