Update eventlistener podSeurityContext to adhere to restricted pod se…

[kristofferchr]

Solves issue #1739

Currently, when el-security-context is enabled, the securityContext for EventListener in the pod template only sets runAsNonRoot. Expanding podTemplate securityContext can make it easier to fulfill restricted pod security standards.

Makes it possible to have a default securityContext set for injected sidecar containers that does not fulfill restricted pod security standars. An example of this is injected istio containers.

Changes

Alter podSecurityContext to include seccompProfile, runAsUser, runAsGroup and fsGroup when set-security-context is set.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• [ ] Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• [x] Has Tests included if any functionality added or changed
• [x] Follows the commit message standard
• [x] Meets the Tekton contributor standards (including functionality, content, code)
• [x] Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• [x] Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• [ ] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

Eventlistener podTemplate now includes securityContext settings: seccompProfile, runAsUser, runAsGroup, and fsGroup when flag el-security-context is true. 

[tekton-robot]

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• :white_check_mark: login: kristofferchr / name: Kristoffer Moberg Christensen (53e47bae8ed7c7d1733f638be2e3437193a2fea9)

[kristofferchr]

/kind feature

[kristofferchr]

/retest

[kristofferchr]

/retest

[tekton-robot]

@kristofferchr: PR needs rebase.

Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.