[#303]fix-Failed_to_get_the_dns_service_IP_list

[carryaimp]

1. Added the logic for determining whether the obtained DNS IP address list is empty
2. Get and set the label of the default selected DNS service through the environment variable

[codecov]

Codecov Report

Merging #306 (8ea6517) into main (8f17d25) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #306   +/-   ##
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  Coverage   40.00%   40.00%           
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  Files           8        8           
  Lines         555      555           
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  Hits          222      222           
  Misses        326      326           
  Partials        7        7           

Flag	Coverage Δ
unittests	40.00% <ø> (ø)

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[ii2day]

@carryaimp We have a special method to obtain env environment variables. Refer to this file to set env.https://github.com/kdoctor-io/kdoctor/blob/main/pkg/types/agent_config.go

[carryaimp]

@ii2day It has been modified according to the specifications

[ii2day]

@weizhoublue lgtm