Resolve "Create secrets failed for Auto DevOps with latest stable of kubectl"

What does this MR do?

Backport gitlab-ci-yml!128 (merged) - Fix kubectl version to 1.8.6 ... This commit extracts kubectl, helm and codeclimate versions as CI variables.

kubectl changes from latest stable version to 1.8.6, the other two are just extracted in order to be easily updated; now we can also test tool upgrades overriding CI secret variables.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

See #41376 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #41376 (closed)