deployment: Obtain repair image from AFE for Skylab deployments

Stable image mappings are still in the AFE. Even in the Skylab flow,
repair image must be obtained from the AFE.

BUG=chromium:912977
TEST=python ./site_utils/deployment/prepare/main.py \
         --dry-run ...

Change-Id: If2538cc4e2620c5019753a5a9e9923feccab7a7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/1547173


Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>