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 fix this problem. Refer patch set [1]

[1] https://review.openstack.org/#/c/189569

Change-Id: I6c0b896a6de1b7193dd4b77b6bc4433d0c75732d

tox.ini

@@ -15,6 +15,7 @@ commands =
   rm -f .testrepository/times.dbm
   find . -type f -name "*.py[c|o]" -delete
   ostestr --concurrency=6 {posargs}
+passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:pep8]
 commands =