Rename CI_COMMIT_REF to CI_COMMIT_SHA

What does this MR do?

• Rename value for CI_COMMIT_REF to CI_COMMIT_SHA when using gitlab-runner exec

Why was this MR needed?

• Variable was renamed from CI_COMMIT_REF to CI_COMMIT_SHA in GitLab 9.0
• CI_COMMIT_REF does not seem to be a valid value

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

I did not add backwards compatibility with the pre-9.0 (CI_BUILD_REF) value as that was not present before and none of the other deprecated CI_BUILD_ values are supported.

Does this MR meet the acceptance criteria?

• [N/A] Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

N/A