Use appropriate rspec wait for async requests with Vue

What does this MR do?

• Switch to appropriate wait_for_vue_resource when trying to wait for Vue reseource in a service.

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

• Related to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1857

Why was this MR needed?

• wait_for_ajax doesn't track Vue resource requests and we are waiting for a Vue resource in a service in that test.
• Adds some more peace of mind to the tests (wasn't failing explicitly without this)

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together