Use JSON only when getting user entries in kubeconfig

[yuxiang-zhang]

Issue #, if available:
#7719, #7723, #7724

Description of changes:
Hard-code json as the output format when getting user entry in kubeconfig.

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[codecov-commenter]

Codecov Report

Merging #7726 (ba2f1ec) into develop (8582913) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           develop   #7726      +/-   ##
==========================================
- Coverage     0.08%   0.08%   -0.01%     
==========================================
  Files          206     206              
  Lines        16486   16488       +2     
==========================================
  Hits            14      14              
- Misses       16472   16474       +2     

Impacted Files	Coverage Δ
awscli/customizations/eks/get_token.py	0.00% <0.00%> (ø)
awscli/customizations/eks/update_kubeconfig.py	0.00% <ø> (ø)

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[tim-finnigan]

@yuxiang-zhang or @stealthycoin can we get your thoughts on this comment: #7719 (comment) ?

I'm not sure this is a complete fix then. New (greenfield) users would be covered, but anyone would else would still have this issue. Since the error message doesn't give any hints on cause they would have to search around, find this issue, and then apply fixes themselves. Unless I'm misunderstanding something.

I would classify this as still open.

Suggestion: Ignore user-set formatting variables when not in an interactive tty?