This change just updates the tox.ini file to call stestr directly
instead of going through ostestr.
For more background you can read the ML posting:
https://goo.gl/TFvcxy
So now you won't need `--n` to run single tests, and you can also
supply regex directly without even the `--`. For example:
`tox -epy27 $regex`
Change-Id: I75d4ca19f87fe1930cf54b554b7e232632a4a21c