playground: allow a pure TiKV cluster

[sticnarf]

What problem does this PR solve?

I usually need a TiKV environment that is not polluted by TiDB. But the TiDB number check in playground prevents me from it!

What is changed and how it works?

Do not check TiDB number in playground.

Check List

Tests

• Manual test (add detailed scripts or steps below)

tiup playground --db 0 works \o/

Release notes:

Allow creating a pure TiKV cluster in TiUP playground.

[codecov-io]

Codecov Report

Merging #926 (076cb5b) into master (fa1a80b) will decrease coverage by 3.83%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #926      +/-   ##
==========================================
- Coverage   55.22%   51.39%   -3.84%     
==========================================
  Files         261      261              
  Lines       19254    19253       -1     
==========================================
- Hits        10633     9895     -738     
- Misses       6918     7744     +826     
+ Partials     1703     1614      -89     

Flag	Coverage Δ
cluster	38.26% <ø> (-5.27%)	⬇️
dm	24.10% <ø> (-0.02%)	⬇️
integrate	46.22% <0.00%> (-3.85%)	⬇️
playground	20.23% <0.00%> (+<0.01%)	⬆️
tiup	17.14% <ø> (ø)
unittest	21.64% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
components/playground/playground.go	41.16% <0.00%> (+0.07%)	⬆️
components/cluster/command/check.go	6.37% <0.00%> (-72.82%)	⬇️
pkg/cluster/task/limits.go	0.00% <0.00%> (-68.75%)	⬇️
pkg/cluster/task/sysctl.go	0.00% <0.00%> (-66.67%)	⬇️
components/cluster/command/audit.go	27.27% <0.00%> (-54.55%)	⬇️
pkg/cluster/operation/check.go	0.00% <0.00%> (-51.95%)	⬇️
pkg/cluster/task/rmdir.go	0.00% <0.00%> (-50.00%)	⬇️
pkg/cluster/operation/operation.go	34.78% <0.00%> (-43.48%)	⬇️
components/cluster/command/rename.go	25.00% <0.00%> (-41.67%)	⬇️
components/cluster/command/stop.go	38.46% <0.00%> (-30.77%)	⬇️
... and 23 more

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[sticnarf]

PTAL @lucklove

[lucklove]

LGTM

[AstroProfundis]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 860
• 916
• 922

[ti-srebot]

/run-all-tests