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.

See Also: https://review.openstack.org/#/c/191600

Change-Id: I0045cb2253a4d102196fdab277462891a886fe3a
Co-Authored-By: Yuiko Takada <e380c54048b41940363be7c59d4b48a5776480a4@mxn.nes.nec.co.jp>
Closes-Bug: #1465086

openstack/python-magnumclient