switch trigger sa ref from global to namespace scoped

[gabemontero]

Changes

So in getting caught up on some thing related to the trigger crd tep, I came across this type and #628

It appears to me that to some extent there was a simple copy done in #628 of https://github.com/tektoncd/triggers/blob/master/pkg/apis/triggers/v1alpha1/event_listener_types.go#L77-L92
including my verbose comment and TODO around the ServiceAccount field.

Now, as already indicated in tektoncd/community#148 and the use of a string type serviceAccountName, as well in the WG discussion, any SA assoicated with a Trigger should be in the same namespace as the trigger

So why make this correction now vs. just waiting until the TEP is implemented?

@dibyom 's comment #628 (comment) is what motivated me to submit this for consideration.

If the "run CLI tool" item noted there drops with sufficient time before the tep does, then we don't want the precedent of triggers using SAs from other namespaces being established.

Thoughts @khrm @dibyom @dorismeixing ?

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Release Notes

[gabemontero]

BTW, reducing the scope of https://github.com/tektoncd/triggers/blob/master/pkg/apis/triggers/v1alpha1/event_listener_types.go#L91 could also be a consderation, though maybe that is a bit muddied by any pre tektoncd/community#148 attempts to have a single event listener in one namespace manage triggers / interceptors in multilpe namespaces

My thought was to tackle that possible discussion separately, but don't feel strongly about that.

[gabemontero]

/assign @dibyom
/assign @khrm

[khrm]

/lgtm

[dibyom]

SGTM, the CLI basically ignores the SA field at the moment but this makes it future proof

/approve

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