[TiDB Dashboard] Playground deploys seperated dashboard server by default. Other improvements.

[SabaPing]

What problem does this PR solve?

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• 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

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 40.02%. Comparing base (6320e3c) to head (48d02b1).
Report is 163 commits behind head on master.

Files	Patch %	Lines
pkg/cluster/spec/dashboard.go	0.00%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2091       +/-   ##
===========================================
- Coverage   56.75%   40.02%   -16.73%     
===========================================
  Files         313      312        -1     
  Lines       33102    33094        -8     
===========================================
- Hits        18784    13244     -5540     
- Misses      12091    18037     +5946     
+ Partials     2227     1813      -414     

Flag	Coverage Δ
cluster	28.00% <0.00%> (-17.23%)	⬇️
dm	25.92% <0.00%> (-0.43%)	⬇️
playground	15.29% <0.00%> (-<0.01%)	⬇️
tiup	16.54% <0.00%> (-0.03%)	⬇️
unittest	?

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[ti-chi-bot]

@SabaPing: PR needs rebase.

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