Add CROS_VENV_CACHE to allow alternate path

BUG=chromium:830765
TEST=None

Change-Id: I594402a91eff18dec9f781cad8b3a7ebe245f209
Reviewed-on: https://chromium-review.googlesource.com/1025012
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Keith Haddow <haddowk@chromium.org>
(cherry picked from commit ec4ec1cc25376e8c752d92b34c0f5d282d059af0)

venv/cros_venv/venvlib.py

diff --git a/venv/cros_venv/venvlib.py b/venv/cros_venv/venvlib.py
index 408543e..cd6f097 100644
--- a/venv/cros_venv/venvlib.py
+++ b/venv/cros_venv/venvlib.py
@@ -272,7 +272,8 @@
 
     Returns absolute path.
     """
-    return os.path.expanduser('~/.cache/cros_venv')
+    return os.environ.get('CROS_VENV_CACHE',
+                          os.path.expanduser('~/.cache/cros_venv'))
 
 
 def _makedirs_exist_ok(path):