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: I685ddbc6bb6e0fe25c308c35a7581785eebe3629

tox.ini

@@ -7,6 +7,7 @@ envlist = functional,pep8
 install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
+passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:pep8]
 commands =