Switch from ostestr to stestr

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

doc/source/contributor/testing.rst

@@ -103,10 +103,13 @@ Or all tests in the test_volume.py file::
 
 You may also use regular expressions to run any matching tests::
 
-    tox -epy27 -- -r test_volume
+    tox -epy27 -- test_volume
 
-For more information on these options and how to run tests, please see the
-`ostestr documentation <https://docs.openstack.org/os-testr/latest/>`_.
+Additionally, when running a single test, or test-file, the `-n` argument is no
+longer required, however it may perform faster if included.
+
+For more information on these options and details about stestr, please see the
+`stestr documentation <http://stestr.readthedocs.io/en/latest/MANUAL.html>`_.
 
 Gotchas
 -------

tox.ini

@@ -18,13 +18,13 @@ install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstac
 
 deps = -r{toxinidir}/test-requirements.txt
 
-# By default ostestr will set concurrency
+# By default stestr will set concurrency
 # to ncpu, to specify something else use
 # the concurrency=<n> option.
 # call ie: 'tox -epy27 -- --concurrency=4'
 commands =
   find . -type f -name "*.pyc" -delete
-  ostestr {posargs}
+  stestr run '{posargs}'
 
 whitelist_externals =
   bash