MCO-1284: MachineOSConfig: set ImageSecretObjectReference to optional

[yuqi-zhang]

Small update to the TP field to optional, so the MCO can use cluster pullsecrets by default.

[openshift-ci-robot]

@yuqi-zhang: This pull request references MCO-1284 which is a valid jira issue.

Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the story to target the "4.18.0" version, but no target version was set.

In response to [this](https://github.com/openshift/api/pull/2089): >Small update to the TP field to optional, so the MCO can use cluster pullsecrets by default. Instructions for interacting with me using PR comments are available [here](https://prow.ci.openshift.org/command-help?repo=openshift%2Fapi). If you have questions or suggestions related to my behavior, please file an issue against the [openshift-eng/jira-lifecycle-plugin](https://github.com/openshift-eng/jira-lifecycle-plugin/issues/new) repository.

[openshift-ci[bot]]

Hello @yuqi-zhang! Some important instructions when contributing to openshift/api: API design plays an important part in the user experience of OpenShift and as such API PRs are subject to a high level of scrutiny to ensure they follow our best practices. If you haven't already done so, please review the OpenShift API Conventions and ensure that your proposed changes are compliant. Following these conventions will help expedite the api review process for your PR.

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: yuqi-zhang Once this PR has been reviewed and has the lgtm label, please assign bparees for approval. For more information see the Kubernetes Code Review Process.

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[yuqi-zhang]

/retest-required

[openshift-ci[bot]]

@yuqi-zhang: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-aws-serial-techpreview	25814d6c9e98b0907ed6e3da26880f3fd440b3f7	link	true	/test e2e-aws-serial-techpreview

Full PR test history. Your PR dashboard.

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-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository. I understand the commands that are listed [here](https://go.k8s.io/bot-commands).