Fixed tox.ini to run py35 tests

* Let tox auto handles the setting for LANG variables in order to
  fix py35 tox venv creation and installation.
* Use OS_TEST_PATH in tox variables and re-use it

Change-Id: I534ed66d154b6b05a03a06eb0baa0ff23d851f4e

tox.ini

@@ -7,13 +7,11 @@ skipsdist = True
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
-         LANG=en_US.UTF-8
-         LANGUAGE=en_US:en
-         LC_ALL=C
+         OS_TEST_PATH=./refstack_client/tests/unit
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
-  stestr --test-path ./refstack_client/tests/unit run {posargs}
+  stestr run {posargs}
 distribute = false
 
 [testenv:pep8]