Change variable for istio appgw cert

Infrastructure Submissions:

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• [YES] Have you added an explanation of what your changes do and why you'd like us to include them?
• [YES] I have updated the documentation accordingly.
• [NA] I have added tests to cover my changes.
• [YES] All new and existing tests passed.
• [YES] I have formatted the terraform code. (terraform fmt -recursive && go fmt ./...)

Current Behavior or Linked Issues

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These changes are needed to separate Istio and AGIC BYOC certificate support.

Does this introduce a breaking change?

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• [NO]

MR Guildelines

• Paste TF Plan for the MR.
• Pre-Merge pipeline should be run before merging. (Azure team)
• Does the module exists for new resource.
• Is there a new variable added in the MR. (Don’t use library variables and use locals)

Other information

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