Tweak kubernetes_service_spec for EE compatibility

What does this MR do?

Allows spec/models/project_services/kubernetes_service_spec.rb to be shared between CE and EE.

Backport for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10023.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6516

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer
• Merge https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10023 before merging

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6516