test_plan_generator: allow local override of config dir

This will allow people to use custom config settings for local
validation of the test plan generator.

Bug: 1130018
Change-Id: Id902fac9601453a8c153c7a2ba49e45a186a9e2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2420306
Reviewed-by: David Burger <dburger@chromium.org>
Commit-Queue: Sean Abraham <seanabraham@chromium.org>
Tested-by: Sean Abraham <seanabraham@chromium.org>
1 file changed