Revert "Revert "Install virtualenv for CentOS via distro packages""
Revert "Revert "Install virtualenv for CentOS via distro packages""

This reverts commit fd585081adff0f7885413f270b2de73e721a8996.

The failures when trying to install pip for CentOS were not due to
the pip/setuptools/wheel packages, but instead due to the
inclusion of the virtualenv package in the list. With the dependent
patch, that is removed, so pip/setuptools/wheel should install
just fine.

Change-Id: I8648dc3dd3ee04ac31a9497bf6a8c677374f1afc
Depends-On: https://review.openstack.org/587798

openstack/openstack-ansible-openstack_hosts,openstack/openstack-ansible-openstack_hosts