Use system.DefaultNamespace for interceptor urls

[dibyom]

Changes

SYSTEM_NAMESPACE is set to the namespace of the EventListener by the reconciler
which means that with the current logic the interceptor url is wrong. Use
default namespace to always use tekton-pipelines as the namespace. This is
hardcoded and the logic is temporary till we implement #868.

/kind bug

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[wlynch]

/lgtm

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[MarcelMue]

@dibyom Can you explain why this was necessary? This currently completely breaks the usecase of running in a different namespace because it will always resolve the service name into a wrong namespace.

[dibyom]

Can you explain why this was necessary? This currently completely breaks the usecase of running in a different namespace because it will always resolve the service name into a wrong namespace.

I thought SYSTEM_NAMESPACE was the namespace Triggers was installed in but it turns out we (also) use it for the namespace that the Event Listener was installed in :(

Tracking in #923