[create_venv] Implement compatibility improvements for RHEL 7.4
[create_venv] Implement compatibility improvements for RHEL 7.4

On RHEL 7.4, which is used for OSP10 < z8 deployments, the version
of virtualenv used does not support the --no-download CLI parameter.

In this patch we adjust the create_venv role to use the older
--never-download parameter which is valid across a broader range of
releases.

We also implement an upgrade of pip in the virtualenv for RHEL 7.4
so that it is upgraded to a version that is capable of using
constraints.

JIRA: UPG-2144

Change-Id: I7549388f1a07735c3671a1835f52414333d5f250

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared