Add ImagePullSecrets field when create Deployment by EventListener

[lx1036]

Changes

fixed #1448

when submit EventListener like this with imagePullSecrets now:

apiVersion: triggers.tekton.dev/v1beta1
kind: EventListener
metadata:
  name: fission-ci-listener-test
spec:
  serviceAccountName: tekton-triggers-example-sa
  resources:
    kubernetesResource:
      spec:
        template:
          spec:
            imagePullSecrets:
              - name: docker-login
  triggers:
    - interceptors:
        - ref:
            name: cel
          params:
            - name: filter
              value: 'body.gitUrl != "" && body.gitRevision != "" && body.oauth != ""'
      bindings:
        - ref: fission-ci-binding-test
      template:
        ref: fission-ci-template-test

it report errors like:

$ k apply -f eventlister.yaml -n tekton-run
Error from server (BadRequest): error when creating "eventlister.yaml": admission webhook "validation.webhook.triggers.tekton.dev" denied the request: validation failed: must not set the field(s): spec.resources.kubernetesResource.spec.template.spec.imagePullSecrets

this PR resolve the issue that deployment with imagePullSecrets by EventListener.

Release Notes

None

[tekton-robot]

@lx1036: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[linux-foundation-easycla]

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[tekton-robot]

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[tekton-robot]

Hi @lx1036. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[khrm]

/ok-to-test

[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/reconciler/eventlistener/resources/deployment.go	100.0%	98.6%	-1.4

[khrm]

Please sign the cla