fix logic error: SetDiscoveryConfig should be called in both local mode and non-local mode

[huangchenzhao]

What type of PR is this?

/kind bug

What this PR does / why we need it:

In #2449, SetDiscoveryConfig is modified, in which this function is only called in non-local mode for yurtadm join. as the following figure.

However, when use in local mode, yurtadm also need call SetDiscoveryConfig to configure discovery file, then kubeadm-config contains this discovery to perfom joining a node.

Which issue(s) this PR fixes:

Fixes #2449

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 47.16981% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.09%. Comparing base (3f83586) to head (e9fcfa7).
⚠️ Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/yurtadm/cmd/join/phases/prepare.go	47.16%	28 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2471      +/-   ##
==========================================
+ Coverage   42.96%   43.09%   +0.12%     
==========================================
  Files         396      396              
  Lines       26298    26337      +39     
==========================================
+ Hits        11300    11350      +50     
+ Misses      13941    13930      -11     
  Partials     1057     1057              

Flag	Coverage Δ
unittests	43.09% <47.16%> (+0.12%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[huangchenzhao]

@zyjhtangtang This PR is originally for fixing the calling logic of function SetDiscoveryConfig. However, for passing Codecov, I refactor prepare.go with interface abstraction and add prepare_test.go. Please take a look, thank you.