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: Iccbd680731592abab4980a51aa97c60d6ef95005

tox.ini

@@ -13,7 +13,7 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
   find {toxinidir} -type f -not -path '{toxinidir}/.tox/*' -not -path '*/__pycache__/*' -name '*.py[c|o]' -delete
-  stestr run '{posargs}'
+  stestr run {posargs}
   stestr slowest
 
 whitelist_externals =
@@ -58,7 +58,7 @@ setenv =
 commands =
   coverage erase
   find {toxinidir} -not -path '{toxinidir}/.tox/*' -not -path '*/__pycache__/*' -name '*.py[c|o]' -delete
-  stestr run '{posargs}'
+  stestr run {posargs}
   coverage combine
   coverage html -d cover
   coverage xml -o cover/coverage.xml