Adds support loading the machine-global config file to pick up
location of the admission plugin binary. See the bug for details.

BUG=1310363
R=iannucci@chromium.org
CC=akashmukherjee@google.com

Change-Id: I7a0cec6a994b79929e204766d32e0f9649b9dc8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3736694Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>