Migrate unit tests from testr to stestr

Note the interface doesn't actually have unit test.  This just migrates
away from the deprecated testr module.

Change-Id: I02cd48778bcbca8f6d05dc83d96d1e44852d1da3

.stestr.conf

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

test-requirements.txt

@@ -1,2 +1,2 @@
-flake8>=2.2.4,<=2.4.1
+flake8>=2.2.4
-os-testr>=0.4.1
+stestr>=2.2.0

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py34,py35,py36
+envlist = pep8,py34,py35
 skipsdist = True
 # NOTE(beisner): Avoid build/test env pollution by not enabling sitepackages.
 sitepackages = False
@@ -11,7 +11,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
 install_command =
   pip install {opts} {packages}
-commands = ostestr {posargs}
+commands = stestr run {posargs}
 
 [testenv:py34]
 basepython = python3.4
@@ -27,6 +27,13 @@ deps = -r{toxinidir}/test-requirements.txt
 # https://github.com/juju/charm-tools/issues/249
 commands = /bin/true
 
+[testenv:py3]
+basepython = python3
+deps = -r{toxinidir}/test-requirements.txt
+# TODO: Need to write unit tests then remove the following command.
+# https://github.com/juju/charm-tools/issues/249
+commands = /bin/true
+
 [testenv:pep8]
 basepython = python3
 deps = -r{toxinidir}/test-requirements.txt