Fix the creation of staging_dir for trybot
Fix the creation of staging_dir for trybot

This change allows the creation of staging_dir using a multi-level rel_path.
This is necessary for trybot runs.

BUG=chromium-os:32132
TEST=test download with a local dev server setup

Change-Id: I0edf994f731e9851dd27b42c200b553664cdebd1
Reviewed-on: https://gerrit.chromium.org/gerrit/26246
Commit-Ready: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Reviewed-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>

coreos/dev-util,mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util