Improve variables support

What does this MR do?

These are random changes extracted from other code that I work on now: https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2563.

This slightly improves variable support, ensuring, that we always pass strings.

This also slightly improves KubernetesNamespace ensuring that namespace and service-account-token are always set.

What are the relevant issue numbers?

None yet

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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.