Introduce a more graceful way to check labels

[lucklove]

Fix #838

Signed-off-by: lucklove gnu.crazier@gmail.com

What problem does this PR solve?

For imported clusters, it will report missing label when executing tiup cluster display since the server.labels is missing in meta.yaml.

What is changed and how it works?

Read labels config from PD server instead of meta.yaml.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release notes:

NONE

[codecov-io]

Codecov Report

Merging #843 into master will increase coverage by 0.11%.
The diff coverage is 69.04%.

@@            Coverage Diff             @@
##           master     #843      +/-   ##
==========================================
+ Coverage   52.99%   53.10%   +0.11%     
==========================================
  Files         261      261              
  Lines       18987    19014      +27     
==========================================
+ Hits        10062    10098      +36     
+ Misses       7367     7357      -10     
- Partials     1558     1559       +1     

Flag	Coverage Δ
#cluster	45.06% <69.04%> (+0.24%)	⬆️
#dm	25.30% <0.00%> (+0.02%)	⬆️
#integrate	47.98% <69.04%> (+0.19%)	⬆️
#playground	22.17% <0.00%> (-0.06%)	⬇️
#tiup	10.78% <0.00%> (-0.03%)	⬇️
#unittest	20.94% <66.66%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
pkg/cluster/manager.go	66.03% <25.00%> (-0.01%)	⬇️
pkg/cluster/api/pdapi.go	53.84% <66.66%> (+2.55%)	⬆️
pkg/cluster/spec/spec.go	87.84% <70.00%> (-0.73%)	⬇️
pkg/cluster/spec/validate.go	95.41% <84.61%> (+1.90%)	⬆️
pkg/cluster/api/dmapi.go	60.00% <0.00%> (+1.73%)	⬆️

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