Accept an optional testing ssh private key

BUG=brillo:1200
TEST=unit tests

Change-Id: Ifbba4eb334ce919a96b5ee5dfce1eefba3edcc03
Reviewed-on: https://chromium-review.googlesource.com/286934
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Daniel Wang <wonderfly@google.com>
Tested-by: Daniel Wang <wonderfly@google.com>
8 files changed