Switch unit test tox jobs to use ostestr

Now that we've deprecated all our bash wrappers for running tests
we shouldn't be using them for running unit tests either. This commit
updates the unit test tox job definitions so that we no longer are
relying on deprecated code in the gate and we can remove the wrappers
when the time is right.

Change-Id: I127786537c0792456d26b13fc3a2044afc4e93a1

tox.ini

@@ -26,7 +26,7 @@ deps =
     -r{toxinidir}/test-requirements.txt
 commands =
     find . -type f -name "*.pyc" -delete
-    bash tools/pretty_tox.sh '{posargs}'
+    ostestr {posargs}
 
 [testenv:genconfig]
 commands = oslo-config-generator --config-file tempest/cmd/config-generator.tempest.conf