Add passenv to use with tox to forward http proxy settings

Without adding passenv http proxy settings in tox.ini, the unit
tests can not reach the internet and tests can not be executed
behind proxy

Change-Id: I8a723a70c5aba8f21c75f704048d6fe1010756fb

tox.ini

@@ -9,6 +9,7 @@ install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
+passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'