MESH-6019 : Make ingress gateway selection consistent.

[virajrk]

Checklist

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• I've read and agree to the project's contribution guidelines.
• I'm requesting to pull a topic/feature/bugfix branch.
• I checked that my code additions will pass code linting checks and unit tests.
• I updated unit and integration tests (if applicable).
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Description

Currently admiral load ingress gateway via two point and two different selector getting used.

• Name of Kubernetes service
• Label selector

Moving this only label selector.

Thank you!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 22.22222% with 7 lines in your changes missing coverage. Please review.

Project coverage is 76.57%. Comparing base (e885c76) to head (4c497b6).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
admiral/pkg/controller/common/common.go	0.00%	5 Missing ⚠️
admiral/pkg/clusters/service_handler.go	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
- Coverage   76.76%   76.57%   -0.19%     
==========================================
  Files          75       75              
  Lines       10300    10303       +3     
==========================================
- Hits         7907     7890      -17     
- Misses       1988     2004      +16     
- Partials      405      409       +4     

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