Migrate to stestr

This patch updates tox.ini to directly call stestr instead of ostestr
[1]. ostestr is no longer installed by default by infra.

For more info on stestr, please refer to [2].

[1] http://lists.openstack.org/pipermail/openstack-dev/2017-September/122135.html
[2] http://stestr.readthedocs.io/en/latest/MANUAL.html

Depends-On: https://review.openstack.org/#/c/550282/
Change-Id: Ie6a275170ec6faf0db3976410660f52078710a54

tools/ostestr_compat_shim.sh

@@ -1,8 +0,0 @@
-#!/bin/sh
-
-# preserve old behavior of using an arg as a regex when '--' is not present
-case $@ in
-  (*--*) ostestr $@;;
-  ('') ostestr;;
-  (*) ostestr --regex "$@"
-esac

tox.ini

@@ -13,7 +13,8 @@ whitelist_externals = find
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
-  {toxinidir}/tools/ostestr_compat_shim.sh {posargs}
+  stestr run '{posargs}'
+  stestr slowest
 
 [testenv:api-ref]
 # This environment is called from CI scripts to test and publish