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.

Change-Id: If43dfb3cf428b8c51d1755b404b97edfc1999856

tox.ini

@@ -7,6 +7,7 @@ envlist = py26,py27,py34,pep8
 usedevelop = True
 install_command = pip install --allow-external -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
+passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =