Add --cache-dir argument to gclient.
Add --cache-dir argument to gclient.

There are some usecases where it is simpler to pass path to the 
cache by an environment variable or command-line argument 
than to modify the gclient config file.

With this change:

 * GCLIENT_CACHE_DIR environment variable overrides cache_dir variable
   from .gclient file.

 * --cache-dir argument moved from gclient config to all gclient
   commands and overrides values from .gclient file and environment
   variable.

R=6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org

Bug:
Change-Id: Iae107f1d5f02a2eee1896d0d3108dc20a83d597f
Reviewed-on: https://chromium-review.googlesource.com/596089
Reviewed-by: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org>
Reviewed-by: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>
Commit-Queue: Aleksandr Derbenev <c47ac41ca5596616675b88339e7b796c623bf3b6@yandex-team.ru>

CoherentLabs/depot_tools,CoherentLabs/depot_tools