enhance output instuction when creating cc cluster with legacy config

[hongtaiwang]

What this PR does / why we need it

When creating cluster using cluster yaml, the validation does not read configurations from the yaml context. So if none custom flags are explicitly set in this step, tanzu cli will validate with defualt value

ALL customized flags are required to set when creating cluster with a cc cluster yaml.

Which issue(s) this PR fixes

Fixes #

Describe testing done for PR

Release note

Additional information

Special notes for your reviewer

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/4511/20230322073912/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[codecov]

Codecov Report

Merging #4511 (8c1df7e) into main (893310a) will decrease coverage by 0.94%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4511      +/-   ##
==========================================
- Coverage   49.87%   48.94%   -0.94%     
==========================================
  Files         453      483      +30     
  Lines       45405    47525    +2120     
==========================================
+ Hits        22647    23260     +613     
- Misses      20612    22064    +1452     
- Partials     2146     2201      +55     

Impacted Files	Coverage Δ
tkg/client/cluster.go	35.86% <100.00%> (ø)

... and 35 files with indirect coverage changes

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[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/4511/20230323015750/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[wjun]

LGTM.