devserver: pass a cache_user to initialize gsutil.

This CL passes an argument cache_user in gsutil initialization. This
'cache_user' is used to create cache directory for gsutil.

BUG=chromium:698304
TEST=emerge chromite to local moblab DUT, call devserver to provision
it. Ran cros flash ssh://.

Change-Id: Ia5789ce18ac4d87e74ccf4f0bb9db7ec535cdccf
Reviewed-on: https://chromium-review.googlesource.com/457861
Commit-Ready: Xixuan Wu <xixuan@chromium.org>
Tested-by: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
3 files changed