Allows to make Istio and VirtualServices optional

[Suresh-Nakkeran]

Signed-off-by: Suresh-Nakkeran suresh.n@ideas2it.com

What this PR does / why we need it:
This PR allows followining:

• Istio is optional for Serverless mode and other networking layer Knative supported can be integrated with KServe such as Contour.
• KServe Serverless installation can fully run in mesh mode without knative local gateway.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1336

Type of changes
Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Feature/Issue validation/testing:

Please describe the tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
Please also list any relevant details for your test configuration.

• When DisableIstioVirtualHost is true, KServe does not create the top level virtual service thus Istio is no longer required for serverless mode.
• When DisableIstioVirtualHost is true, KServe works with Contour.
• When DisableIstioVirtualHost is false, KServe works like before.

Special notes for your reviewer:

1. The Service url in status returns the InferenceService component ksvc url instead of the url with virtual host(isvc name) as top level vs is now disabled, e.g when transformer is added to the InferenceService, the url returns the transformer ksvc url otherwise it returns the predictor ksvc url.

Checklist:

• Have you added unit/e2e tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?

Release note:

`DisableIstioVirtualHost` is introduced in the configmap to disable top level Istio virtual service, Istio is no longer required for serverless mode.

[Suresh-Nakkeran]

/retest

[yuzisun]

/lgtm
/approve

[kserve-oss-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Suresh-Nakkeran, yuzisun

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[rachitchauhan43]

@Suresh-Nakkeran : 1qq, say we go with disabling the top level virtual service and now deploy an ISVC with both transformer and a predictor.

1. Now when i make a call to my ISVC, I will have to use transformer’s url right ?
2. Will Kserve still support routing request from transformer to predictor or not ?

Is it possible to add a diagram of what was there earlier with Top Level VS and what can now be achieved with disabling top level VS ?