Replace ostestr with stestr in testing framework.

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

Change-Id: I8a47c6d9615c3e85d72bbcba8941619512c2da72

.stestr.conf

+[DEFAULT]
+test_path=./unit_tests
+top_dir=./

.testr.conf

-[DEFAULT]
-test_command=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
-             OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
-             OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-60} \
-             ${PYTHON:-python} -m subunit.run discover -t ./ ./unit_tests $LISTOPT $IDOPTION
-
-test_id_option=--load-list $IDFILE
-test_list_option=--list

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

@@ -12,7 +12,7 @@ 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_*