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: I5b47eff597f3a1a4da2dd8afba08c508d8ebeac9

stackforge/fuel-stats,stackforge/fuel-stats,stackforge/fuel-stats