At periodic-tripleo-ci-centos-7-ovb-1ctlr_1comp-featureset020-ocata No package matching 'python-stestr'

[Quique Llorente]

Installing the packages for tempest fails at ocata, ocata doesn't have python-stestr.

https://review.rdoproject.org/jenkins/job/periodic-tripleo-ci-centos-7-ovb-1ctlr_1comp-featureset020-ocata/230/console

05:28:58 TASK [validate-tempest : Install packages to generate subunit results] *********
05:28:58 Wednesday 25 April 2018 05:28:58 +0000 (0:00:14.456) 0:01:08.854 *******
05:29:02 failed: [undercloud] (item=python-stestr) => {"changed": false, "failed": true, "item": "python-stestr", "msg": "No package matching 'python-stestr' found available, installed or updated", "rc": 126, "results": ["No package matching 'python-stestr' found available, installed or updated"]}
05:29:05 ok: [undercloud] => (item=python-subunit)
05:29:15 changed: [undercloud] => (item=subunit-filters)

Fix https://review.openstack.org/#/c/563843/