Add kubeclients stub methods on `KubernetesHelpers`

What does this MR do?

Adds the following kubeclient stub methods into KubernetesHelpers spec

• GET service account
• PUT service account
• GET role binding
• PUT role binding

What are the relevant issue numbers?

Those stub methods are being used in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23922, but since that MR is getting quite big, we've decided to split it into more manageable and independent MR, so they are easier to review.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] 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