Migrate from testr to stestr

Note that the interface doesn't actually have unit tests.  This is to
migrate from the deprecated testr module.

Change-Id: I01866ccf48f09d757f54db2d573d5661f66198c1

.gitreview

@@ -0,0 +1,4 @@
+[gerrit]
+host=review.opendev.org
+port=29418
+project=openstack/charm-interface-websso-fid-service-provider.git

.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
-os-testr>=0.4.1
+flake8>=2.2.4
+stestr>=2.2.0

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27,py34,py35
+envlist = pep8,py35
 skipsdist = True
 # NOTE(beisner): Avoid build/test env pollution by not enabling sitepackages.
 sitepackages = False
@@ -11,19 +11,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
 install_command =
   pip install {opts} {packages}
-commands = ostestr {posargs}
-
-[testenv:py27]
-basepython = python2.7
-deps = -r{toxinidir}/test-requirements.txt
-# TODO: Need to write unit tests then remove the following command.
-commands = /bin/true
-
-[testenv:py34]
-basepython = python3.4
-deps = -r{toxinidir}/test-requirements.txt
-# TODO: Need to write unit tests then remove the following command.
-commands = /bin/true
+commands = stestr run {posargs}
 
 [testenv:py35]
 basepython = python3.5
@@ -43,6 +31,12 @@ 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