Replace ostestr with stestr in testing framework.

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

Change-Id: Ic14787c7fa9ee4936cc8e098511b87a53b93c731

test-requirements.txt

@@ -5,7 +5,7 @@ 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
 # BEGIN: Amulet OpenStack Charm Helper Requirements
 # Liberty client lower constraints

tox.ini

@@ -13,7 +13,7 @@ setenv = VIRTUAL_ENV={envdir}
          AMULET_SETUP_TIMEOUT=5400
 install_command =
   pip install {opts} {packages}
-commands = ostestr {posargs}
+commands = stestr run {posargs}
 whitelist_externals = juju
 passenv = HOME TERM AMULET_* CS_API_*
 
@@ -22,7 +22,7 @@ basepython = python2.7
 changedir = swift_manager
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = ostestr --path . {posargs}
+commands = stestr run {posargs}
 
 [testenv:py35]
 basepython = python3.5
@@ -52,7 +52,7 @@ setenv =
     PYTHON=coverage run
 commands =
     coverage erase
-    ostestr {posargs}
+    stestr run {posargs}
     coverage combine
     coverage html -d cover
     coverage xml -o cover/coverage.xml