Add support for Istio VirtualService delegation

[kazukousen]

the Istio provider now supports VIrtualService delegate.
https://istio.io/latest/docs/reference/config/networking/virtual-service/#Delegate
for example, if rootVS and delegateVS are there, the VirtualService which Flagger creates can achieve to behave as delegateVS.
it might related #673 .

for that, the Hosts and Gateways must accept an empty to be applied as a delegateVS.

[mathetake]

@kazukousen should we add some test in pkg/router/istio_test.go?

[kazukousen]

@mathetake i see, later i will try adding test.

[kazukousen]

In addition to unit testing, i have added e2e testing to ensure that the CRD is actually behaving correctly and delegating as expected.

its test want to switch enable/disable to pilot env PILOT_ENABLE_VIRTUAL_SERVICE_DELEGATE.
however, when running a rolling update of istiod by setting env, running 2 replicas will cause a timeout due to insufficient CI machine specs.

My idea is to keep the number of replicas to 1 by adding the --set values.pilot.rollingMaxSurge=0 --set values.pilot.rollingMaxUnavailable=1 option to the installation by istioctl manifest apply.

If you can accept this, I will try it.

[stefanprodan]

@kazukousen you could also patch istiod and set the memory and cpu requests super low. I am ok with whatever works. Thanks

[kazukousen]

with the istiod cpu and memory requests set low, it works.
however, calling webhook "validation.istio.io" failed.
https://app.circleci.com/pipelines/github/weaveworks/flagger/1692/workflows/0edee968-7cd8-45fa-b155-fd7a3bb398cb/jobs/9084/parallel-runs/0/steps/0-111
i found a workaround that sets --set values.global.configValidation=false. is it ok?

[stefanprodan]

Please update istio in e2e to a version that supports delegation. I guess that’s the reason validation fails.

[kazukousen]

@stefanprodan
i updated istio and it worked well.
consider in istio 1.7.x is that we need to self-install prometheus, added a patch that.
thanks!

[stefanprodan]

@kazukousen can you please undo the kustomization change and use https://istio.io/latest/docs/ops/integrations/prometheus/ in e2e tests.

[kazukousen]

the failed test test/e2e-istio-tests.sh did not reproduce on my laptop...

[stefanprodan]

LGTM

Thanks @kazukousen 🏅