Also makes bot_update use this.

This allows for clank to manually set the gclient config.

BUG=590788

Review URL: https://codereview.chromium.org/1815863002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299398 0039d316-1c4b-4281-b951-d872f2087c98