Add config-defaults configmap to set default values

[savitaashture]

Changes

Adding config-defaults configmap to set default values and as of now serviceAccountName added to config-defaults
Fixes: #682

/cc @dibyom @khrm

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

Triggers now support config-defaults configmap to set default values.

[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/config/default.go	Do not exist	92.3%
pkg/apis/config/store.go	Do not exist	62.5%
pkg/apis/triggers/v1alpha1/event_listener_defaults.go	81.8%	86.7%	4.8

[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/config/default.go	Do not exist	92.3%
pkg/apis/config/store.go	Do not exist	62.5%
pkg/apis/triggers/v1alpha1/event_listener_defaults.go	81.8%	86.7%	4.8

[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/config/default.go	Do not exist	92.3%
pkg/apis/config/store.go	Do not exist	62.5%
pkg/apis/triggers/v1alpha1/event_listener_defaults.go	81.8%	86.7%	4.8

[dibyom]

/lgtm

[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/config/default.go	Do not exist	92.3%
pkg/apis/config/store.go	Do not exist	62.5%
pkg/apis/triggers/v1alpha1/event_listener_defaults.go	81.8%	86.7%	4.8

[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/config/default.go	Do not exist	92.3%
pkg/apis/config/store.go	Do not exist	62.5%
pkg/apis/triggers/v1alpha1/event_listener_defaults.go	81.8%	86.7%	4.8

[dibyom]

/lgtm

[tekton-robot]

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