Replace ostestr with stestr in testing framework.

A system upgrade broke ostestr. We can fix it by just calling stestr
directly.

Change-Id: I68b0f867416ae6f00f626eb16e0ed2da5e592df0

src/test-requirements.txt

@@ -5,6 +5,6 @@ charm-tools>=2.4.4
 coverage>=3.6
 mock>=1.2
 flake8>=2.2.4,<=2.4.1
-os-testr>=0.4.1
+stestr>=2.2.0
 requests>=2.18.4
 git+https://github.com/openstack-charmers/zaza.git#egg=zaza

test-requirements.txt

@@ -4,7 +4,7 @@
 #
 # Lint and unit test requirements
 flake8>=2.2.4,<=2.4.1
-os-testr>=0.4.1
+stestr>=2.2.0
 requests>=2.18.4
 charms.reactive
 mock>=1.2

tox.ini

@@ -25,17 +25,17 @@ commands =
 [testenv:py3]
 basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
-commands = ostestr {posargs}
+commands = stestr run {posargs}
 
 [testenv:py35]
 basepython = python3.5
 deps = -r{toxinidir}/test-requirements.txt
-commands = ostestr {posargs}
+commands = stestr run {posargs}
 
 [testenv:py36]
 basepython = python3.6
 deps = -r{toxinidir}/test-requirements.txt
-commands = ostestr {posargs}
+commands = stestr run {posargs}
 
 [testenv:pep8]
 basepython = python3
@@ -53,7 +53,7 @@ setenv =
     PYTHON=coverage run
 commands =
     coverage erase
-    ostestr {posargs}
+    stestr run {posargs}
     coverage combine
     coverage html -d cover
     coverage xml -o cover/coverage.xml