Add virtualenv requirement to tox.ini
Add virtualenv requirement to tox.ini

We're getting divergence between local test runs and the gate based
on what version of pip is being used by tox, which depends on what
version of virtualenv (which specifies the versions of pip,
setuptools, and wheel) that tox uses in your local environment.  To
eliminate guesswork, tell tox what version of virtualenv it should
use.  Also specify a minversion of tox that can do this.

Change-Id: I01451ec27d3ad6e759636902e497272a89cf6d16

mahak/cinder,mahak/cinder,openstack/cinder,openstack/cinder