cluster: allow editing of lernel_config in tiflash

[AstroProfundis]

What problem does this PR solve?

Fix #1493

Related changes

• Need to cherry-pick to the release branch

Release notes:

NONE

[codecov-commenter]

Codecov Report

Merging #1494 (348b11e) into master (600edd2) will increase coverage by 4.45%.
The diff coverage is n/a.

❗ Current head 348b11e differs from pull request most recent head f9c46ba. Consider uploading reports for the commit f9c46ba to get more accurate results

@@            Coverage Diff             @@
##           master    #1494      +/-   ##
==========================================
+ Coverage   26.26%   30.72%   +4.45%     
==========================================
  Files         267      279      +12     
  Lines       23834    24328     +494     
==========================================
+ Hits         6261     7475    +1214     
+ Misses      16750    15860     -890     
- Partials      823      993     +170     

Flag	Coverage Δ
integrate	21.88% <ø> (+5.68%)	⬆️
playground	13.66% <ø> (?)
tiup	16.20% <ø> (ø)
unittest	22.81% <ø> (ø)

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

Impacted Files	Coverage Δ
pkg/cluster/spec/tiflash.go	53.47% <ø> (ø)
components/playground/instance/process.go	84.90% <0.00%> (ø)
components/playground/instance/tiflash_config.go	100.00% <0.00%> (ø)
components/playground/instance/tikv.go	67.85% <0.00%> (ø)
components/playground/instance/pump.go	0.00% <0.00%> (ø)
components/playground/instance/tikv_config.go	100.00% <0.00%> (ø)
components/playground/instance/drainer.go	0.00% <0.00%> (ø)
components/playground/instance/pd.go	75.00% <0.00%> (ø)
components/playground/instance/instance.go	51.72% <0.00%> (ø)
components/playground/instance/tiflash.go	61.98% <0.00%> (ø)
... and 21 more

---

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[lucklove]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• lucklove

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[AstroProfundis]

/merge

[ti-chi-bot]

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

Commit hash: bb7c177