Add gsutil tool to repo

- use gsutil to download system image files from Google Storage
- since scp from master are very slow, use gsutil on slave machines
to download from GS directly

Change-Id: I817e8c46b6614c37d9100c1510ab6748843985b9
1451 files changed
tree: aac57f428e629c08caac9db3885479d400345790
  1. build/
  2. emu_test/
  3. system_image_uitests/