Convert tox.ini to using stestr

With the upgrade to oslotest 3.6.0 [0], testr no longer works [1].
This is because oslotest no longer requires testr and we don't depend
on it directly in oslo.limit.

[0] d5a3c58f71
[1] 897823fbd6

Change-Id: I4357350b3ad442658ef872c295dbe734a0f2cc9b

.gitignore

@@ -26,6 +26,7 @@ pip-log.txt
 .tox
 nosetests.xml
 .testrepository
+.stestr
 
 # Translations
 *.mo

tox.ini

@@ -8,7 +8,7 @@ deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
-commands = python setup.py testr --slowest --testr-args='{posargs}'
+commands = stestr run --slowest {posargs}
 
 [testenv:py27]
 basepython = python2.7