Configure tracing in e2e tests

[slinkydeveloper]

Problem
Now e2e tests have an hardcoded configuration for pointing to a zipkin instance. Make this value configurable

Persona:
Which persona is this feature for? Knative developer

Exit Criteria
I can configure zipkin to run the e2e tests

[slinkydeveloper]

/assign

[aliok]

This would be nice to have.
When I do something around tracing, I always end up doing some manual config for Zipkin to install it on istio-system namespace etc.

[matzew]

yeah, it would be nice

[matzew]

is it possible to install the zupkin bits onto a different namespace ?

I personally find the hard-coded reference to istio-system (very) confusing

[matzew]

@Harwayne any thoughts, Adam ?

[slinkydeveloper]

I started removing the hardcoded values in pkg: knative/pkg#1448