This reverts commit a85ce75d6b.
The reason for this revert is because it breaks CentOS dsvm jobs:
http://logs.openstack.org/35/332435/6/check/gate-ansible-role-cloud-launcher-dsvm-ansible-func-centos-7/13b3b66/logs/devstacklog.txt.gz
It is due to DevStack removing python-virtualenv by default, causing issues
as CentOS images have virtualenv installed via system-package and pip
by DIB.
Closes-Bug: 1599277
Change-Id: Ib0c01a7b3be32159e7c864a465fe9455e8d651ef