Requirements file missing python-virtualenv
Requirements file missing python-virtualenv

On systems where python-virtualenv is not installed, rally fails
to run. As such adding the package to requirements required for
running rally.

Change-Id: Ib95331bab90d0926750548704ac4ba7e997d72f1
Closes-Bug: #1678047

openstack/rally,openstack/rally,openstack/rally,yeming233/rally,openstack/rally,yeming233/rally