Hard coded istio-system usage on tracing

[matzew]

Describe the bug

Some tracing code has hard coded istio-system dependency:

• https://github.com/knative/eventing/blob/master/pkg/tracing/setup.go
• https://github.com/knative/eventing/blob/master/test/conformance/helpers/tracing/zipkin.go

Expected behavior

I guess no hard coded usage of istio-system

Knative release version
HEAD

Additional context

Perhaps related:

• Monitoring docs depend on istio docs#2290
• Common namespace name for the ingress ?  serving#7197

[slinkydeveloper]

/assign

I can take care of this one

[slinkydeveloper]

Looks like the adapter.Main, used by most sources, uses the hardcoded istio-system config: https://github.com/knative/eventing/blob/master/pkg/adapter/main.go#L118

While other components, like the IMC dispatcher, uses a dynamic config based on config-tracing ConfigMap: https://github.com/knative/eventing/blob/master/pkg/reconciler/inmemorychannel/dispatcher/controller.go#L59