Switch to using stestr

According to Openstack summit session [1] stestr is maintained
project to which all Openstack projects should migrate.
Let's switch it then.

[1] https://etherpad.openstack.org/p/YVR-python-pti

Change-Id: Ifb460199c94d748988d4f0e836e90d1dd6f294bc
Signed-off-by: Chuck Short <chucks@redhat.com>

.stestr.conf

@@ -0,0 +1,3 @@
+[DEFAULT]
+test_path=${OS_TEST_PATH:-./tests}
+top_dir=./

test-requirements.txt

@@ -7,5 +7,5 @@ mock>=2.0.0 # BSD
 coverage!=4.4,>=4.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 prometheus_client
-os-testr>=1.0.0 # Apache-2.0
+stestr>=1.0.0 # Apache-2.0
 docutils>=0.11 # OSI-Approved Open Source, Public Domain

tox.ini

@@ -26,7 +26,7 @@ commands =
 basepython = python2.7
 commands =
   {[testenv]commands}
-  ostestr {posargs}
+  stestr run {posargs}
 
 [testenv:cover]
 basepython = python3