fix config missing default_cluster_name yaml

[xiangtianyu]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What this PR does / why we need it:

fix #1264

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-commenter]

Codecov Report

Merging #1277 (3ddac76) into master (4eede7e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1277   +/-   ##
=======================================
  Coverage   42.72%   42.72%           
=======================================
  Files          73       73           
  Lines        6486     6486           
=======================================
  Hits         2771     2771           
  Misses       3418     3418           
  Partials      297      297           

Impacted Files	Coverage Δ
pkg/config/config.go	66.34% <ø> (ø)

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[tao12345666333]

LGTM