Pass environment variables of proxy to tox
Pass environment variables of proxy to tox

When a development environment is under a proxy, tox is failed even if
environment variables of the proxy are set.

This patch fixes this problem.
Closes-Bug: #1525115

Change-Id: I436e49ab6f87fa7d4bfeaaa915b4ab8e2af6d5b5

stackforge/fuel-plugins,stackforge/fuel-plugins