Pass environment variables of proxy to tox
Pass environment variables of proxy to tox

When a development environment is under a proxy, tox fails even if env
variables are set, this patch will fix this problem.

Change-Id: I8ae234be37937dea804d80a87bb43755c847b4e3

NeCTAR-RC/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,BiznetGIO/horizon,yeming233/horizon,openstack/horizon,noironetworks/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,yeming233/horizon,NeCTAR-RC/horizon,openstack/horizon,openstack/horizon,yeming233/horizon,noironetworks/horizon,yeming233/horizon,NeCTAR-RC/horizon,noironetworks/horizon,BiznetGIO/horizon,ChameleonCloud/horizon,BiznetGIO/horizon,noironetworks/horizon,BiznetGIO/horizon,openstack/horizon