Migrate from testr to stestr module

Note that the interface doesn't have any unit tests yet.

Change-Id: Ifc2f9c0b4a63a2728b8ae6601326f8ccd4887184

.stestr.conf

@@ -0,0 +1,3 @@
+[DEFAULT]
+test_path=./unit_tests
+top_dir=./

test-requirements.txt

@@ -1 +1,2 @@
-flake8>=2.2.4,<=2.4.1
+flake8>=2.2.4
+stestr>=2.2.0

tox.ini

@@ -11,7 +11,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
 install_command =
   pip install {opts} {packages}
-commands = ostestr {posargs}
+commands = stestr run {posargs}
 
 [testenv:py35]
 basepython = python3.5
@@ -19,6 +19,12 @@ deps = -r{toxinidir}/test-requirements.txt
 # TODO: Need to write unit tests then remove the following command.
 commands = /bin/true
 
+[testenv:py3]
+basepython = python3
+deps = -r{toxinidir}/test-requirements.txt
+# TODO: Need to write unit tests then remove the following command.
+commands = /bin/true
+
 [testenv:pep8]
 basepython = python3
 deps = -r{toxinidir}/test-requirements.txt