Switch test runner to ostestr wrapper

We can use ostestr since developers are more familiar
with using ostestr and are used to seeing the
test outputs with this tool around OpenStack
projects (including openstack/manila).

This wrapper gives us a lot of other advantages [1].

TrivialFix

[1] http://docs.openstack.org/developer/os-testr/ostestr.html

Change-Id: Ibc938827ab281caa905def6e2d74dafda90b9394

tox.ini

@@ -15,7 +15,7 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
   find . -type f -name "*.pyc" -delete
-  python setup.py testr --testr-args='{posargs}'
+  ostestr {posargs}
 
 [testenv:debug]
 commands = oslo_debug_helper -t manilaclient/tests {posargs}