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 is
a new feature above tox 2.0, you can check it from [1].

This patch fix this problem.

[1]. https://tox.readthedocs.org/en/latest/example/basic.html#passing-down-environment-variables

Change-Id: I15615bd3e789a580e3f25f25228851b78197e7a7

steveb/heat,cwolferh/heat-scratch,openstack/heat,openstack/heat,steveb/heat,noironetworks/heat,cwolferh/heat-scratch,noironetworks/heat