Add test builder and default values to triggertemplate params

[savitaashture]

Issue: #549

Changes

1. Added defaults to triggertemplate params
2. Added test builder for the params of type Array

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• Includes tests (if functionality changed/added)
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Release Notes

[tekton-robot]

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[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-triggers-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/triggers/v1alpha1/trigger_template_defaults.go	Do not exist	100.0%

[dibyom]

I don't think we support array type params in trigger templates either: https://github.com/tektoncd/triggers/blob/master/pkg/template/resource.go#L108

#532 should make this more explicit

[savitaashture]

Okay thank you
I will close this PR 👍