Switch from ostestr to stestr

This change 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: I2b9b7fbd60b2cc5eea8af01e20cde5714df5027a

test-requirements.txt

@@ -4,10 +4,8 @@
 
 hacking<0.13,>=0.12.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
-python-subunit>=0.0.18 # Apache-2.0/BSD
 sphinx>=1.6.2 # BSD
 oslotest>=1.10.0 # Apache-2.0
-os-testr>=0.8.0 # Apache-2.0
 testrepository>=0.0.18 # Apache-2.0/BSD
 testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=1.4.0 # MIT

tox.ini

@@ -12,7 +12,7 @@ setenv =
 deps = -r{toxinidir}/test-requirements.txt
 commands =
   find . -type f -name "*.pyc" -delete
-  ostestr {posargs}
+  stestr run {posargs}
 whitelist_externals =
   rm
   find