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.

See:
https://review.openstack.org/#/c/189569

Closes-Bug: #1465086

Change-Id: I76727e598ebb77dc146370b576d6992db3ca1ec9

tox.ini

@@ -11,6 +11,7 @@ setenv =
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
+passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:functional]
 sitepackages = True