Fix broken certificate-authority-data with kubectl >= 1.8.0

What does this MR do?

We are encoding kubernetes CA certificate with Base64.encode64, that introduces newlines each 60 char, using Base64.strict_encode64 ensure a better compatibility.

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

Why was this MR needed?

Stable kubectl is v1.8.0, without this fix AutoDevops fails.

tested on the following kubectl versions:

• 1.6.4
• 1.7.7
• 1.8.0

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

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/38685.

/cc @bikebilly @ayufan