Don't quote {posargs} in tox.ini

Quotes around {posargs} cause the entire string to be combined into one
arg that gets passed to stestr. This prevents passing multiple args
(e.g. '--concurrency=16 some-regex')

Change-Id: Id7eb248942aff41902dc1e8875530cba49fdf1ee

tox.ini

@@ -32,7 +32,7 @@ setenv = VIRTUAL_ENV={envdir}
          NOSE_COVER_HTML_DIR={toxinidir}/cover
          PYTHON=coverage run --source kolla_ansible --parallel-mode
 commands =
-    stestr run '{posargs}'
+    stestr run {posargs}
     coverage combine
     coverage html -d cover
     coverage xml -o cover/coverage.xml