generating ingress DR for VS based routing

[shriramsharma]

Description

What does this change do and why?
Generates the DR which applied to the ingress in order to facilitate VS based cross cluster routing

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 65.11628% with 30 lines in your changes missing coverage. Please review.

Project coverage is 71.59%. Comparing base (14b4735) to head (a01ece8).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
admiral/pkg/clusters/serviceentry.go	4.76%	20 Missing ⚠️
admiral/pkg/clusters/virtualservice_routing.go	90.16%	4 Missing and 2 partials ⚠️
admiral/pkg/controller/common/config.go	0.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
- Coverage   71.65%   71.59%   -0.07%     
==========================================
  Files          66       66              
  Lines        9212     9298      +86     
==========================================
+ Hits         6601     6657      +56     
- Misses       2253     2281      +28     
- Partials      358      360       +2     

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[kpharasi]

Let's move this here: https://github.com/istio-ecosystem/admiral/blob/master/admiral/pkg/util/constants.go?