Unable to test locally patches under review

[Bertrand Souville]

Bug Description:
================

If user tries to run Blazar tests on a local environment (e.g. DevStack) using tox, the tests are not executed.

Steps to reproduce:
===================
Step 1: execute the following tox command (patch under review is https://review.openstack.org/#/c/578637/):

$ tox -e py35 blazar.tests.utils.openstack.test_placement

What happened:
running testr
....
PASSED (id=0)

What is expected:
running testr
...
Ran 19 (+1ls9) tests in 0.489s
PASSED (id=2)
...
Test id Runtime
------------------------------------------------------------------------------------------------ -------
...
blazar.tests.utils.openstack.test_placement.TestPlacementClient.test_create_resource_provider 0.026
...