Unifying proxy variables for tox

This patch adds proxy-related variables to tox command.

Change-Id: I0e4a09d809be92c0dffd948e0b9a99bb3321a1fd

tox.ini

@@ -7,6 +7,7 @@ whitelist_externals = rm
 setenv = VIRTUAL_ENV={envdir}
          LANGUAGE=en_US
          LC_ALL=en_US.utf-8
+passenv=HTTP_PROXY HTTPS_PROXY http_proxy https_proxy NO_PROXY no_proxy
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =