Fixed tox.ini to run py35 tests
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

stackforge/refstack-client,openstack/refstack-client,openstack/refstack-client,stackforge/refstack-client