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 is a
new feature above tox 2.0, you can check it from [1].

This patch fix this problem.

[1]. https://tox.readthedocs.org/en/latest/example/basic.html#passing-down-environment-variables

Change-Id: I9dae04de7053773cb5ba13a809145b86cb149614

tox.ini

@@ -5,7 +5,7 @@ skipsdist = True
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
-passenv = TRACE_FAILONLY GENERATE_HASHES
+passenv = TRACE_FAILONLY GENERATE_HASHES http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 usedevelop = True
 install_command =
                   constraints: {[testenv:common-constraints]install_command}