Fix an issue on running subset of tests

It broke after the switching to stestr [1] since stestr doesn't
have the --regex arguments.

[1] https://review.openstack.org/#/c/571471/

Change-Id: Ied13d3c370bba2a18cd54edae0dc80050e87f07e
Closes-Bug: #1800387

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
-    (*--*) stestr run $@;;
-    ('') stestr run;;
-    (*) stestr run --regex "$@"
-esac

tox.ini

@@ -17,7 +17,7 @@ deps = -r{toxinidir}/requirements.txt
 whitelist_externals = sh
                       find
 commands = find . -type f -name "*.py[c|o]" -delete
-           {toxinidir}/tools/ostestr_compat_shim.sh {posargs}
+           stestr run {posargs}
 
 [testenv:fullstack]
 basepython = python2.7