Fix failed to access pip server when run behind proxy

When we run tempest on the machine behind proxy,the pip command failed to
connect to pypi.python.org to download packages.

Fix it by add another option "passenv" to [testenv] section.

Change-Id: Iedc40a2266be59a2298d5909486a1439f2757b8c
Closes-Bug: #1463978
1 file changed