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 fix this problem.

See:
https://review.openstack.org/#/c/189569

Change-Id: I9f88235c58422c420bbd6ba4a3d8c4a5c60db246

maestro-hybrid-cloud/keystone,himanshu-setia/keystone,dstanek/keystone,roopali8/keystone,jamielennox/keystone,dstanek/keystone,openstack/keystone,cernops/keystone,dims/keystone,vivekdhayaal/keystone,openstack/keystone,jonnary/keystone,ilay09/keystone,mahak/keystone,ajayaa/keystone,idjaw/keystone,maestro-hybrid-cloud/keystone,klmitch/keystone,ajayaa/keystone,jamielennox/keystone,takeshineshiro/keystone,takeshineshiro/keystone,rajalokan/keystone,openstack/keystone,ilay09/keystone,himanshu-setia/keystone,ilay09/keystone,dstanek/keystone,mahak/keystone,jonnary/keystone,mahak/keystone,idjaw/keystone,rajalokan/keystone,rajalokan/keystone,roopali8/keystone,cernops/keystone,dims/keystone,klmitch/keystone,vivekdhayaal/keystone,vivekdhayaal/keystone