Pass environment variables of proxy to tox
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 patch fixes this problem.

Change-Id: I2625f9689d72ea68f8daa1b5e442a75b50fbfa52
Closes-bug: #1524824

NeCTAR-RC/murano,DavidPurcell/murano_temp,NeCTAR-RC/murano,olivierlemasle/murano,DavidPurcell/murano_temp,satish-avninetworks/murano,DavidPurcell/murano_temp,olivierlemasle/murano,NeCTAR-RC/murano,NeCTAR-RC/murano,DavidPurcell/murano_temp,olivierlemasle/murano,satish-avninetworks/murano,olivierlemasle/murano,satish-avninetworks/murano,openstack/murano,satish-avninetworks/murano,openstack/murano