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: If225be953bf78e9f52a3f41797b7746ac823e5cc

Author: Nguyen Van Trung

tox.ini

@@ -54,7 +54,7 @@ basepython = python3.6
 commands = pytest {posargs}
 
 [testenv:cover]
-commands = python setup.py test --coverage --testr-args='{posargs}'
+commands = python setup.py test --coverage --testr-args={posargs}
 
 [testenv:docs]
 commands = python setup.py build_sphinx