Pass proxy environment variable to tox

This will allow test to work if developement environment is under proxy

Change-Id: Id89c57ac455c4b93bcaed8e9166b7a44bad2a25c

tox.ini

@@ -8,6 +8,7 @@ skipsdist = True
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
+passenv = *_proxy *_PROXY
 
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt