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=dpranke@chromium.org

Bug:
Change-Id: Iae107f1d5f02a2eee1896d0d3108dc20a83d597f
Reviewed-on: https://chromium-review.googlesource.com/596089Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Daniel Jacques <dnj@chromium.org>
Commit-Queue: Aleksandr Derbenev <alex-ac@yandex-team.ru>