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: I6a18fa58d85f0d75befedba4bac5cc3c116da73c
(cherry picked from commit 156a71d2dc)

tox.ini

@@ -18,7 +18,7 @@ deps =
        -r{toxinidir}/test-requirements.txt
 commands =
   find . -type f -name "*.pyc" -delete
-  stestr run '{posargs}'
+  stestr run {posargs}
   stestr slowest
 
 [testenv:debug]
@@ -74,7 +74,7 @@ setenv =
 passenv = OS_*
 commands =
   {envdir}/bin/python setup.py install
-  stestr run '{posargs}'
+  stestr run {posargs}
 
 [testenv:genconfig]
 basepython = python3