DefaultDomain should be the cluster's domain and not hardcoded to svc.cluster.local

[dprotaso]

Rather than hardcode the default domain I think we should use the cluster's domain from the knative/pkg helper

serving/pkg/reconciler/route/config/domain.go

Lines 34 to 36 in ea2a6c8

	// DefaultDomain holds the domain that Route's live under by default
	// when no label selector-based options apply.
	DefaultDomain = "svc.cluster.local"

and use

network.GetClusterDomainName()

https://github.com/knative/pkg/blob/db8a35330281c41c7e8e90df6059c23a36af0643/network/domain.go#L46

I don't think we catch this in our e2e tests because we use a fake domain name - https://github.com/knative/serving/blob/ea2a6c8c6166d27b3251469400efed33125cac15/test/config/ytt/core/overlay-config-domain.yaml

FYI @psschwei @nak3

[kauana]

/assign @kauana