Execute tests by class, not randomly

The default test runner behavior is to distribute various classes
across many runners, which is fine. But when you have lots of setup
to ensure an environment is correct for tests in a class, you
generally want to execute that together instead of separately.

As such, set the --parallel-class execution option for stestr.

Change-Id: I5a65accfb7e2033690b2934d874141db7f4bf383

tox.ini

@@ -22,7 +22,7 @@ deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
 commands =
-    stestr run --slowest {posargs}
+    stestr run --slowest --parallel-class {posargs}
 passenv = http_proxy
           HTTP_PROXY
           https_proxy
@@ -67,7 +67,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHON=coverage run --source ironic --omit='*tests*' --parallel-mode
 commands =
   coverage erase
-  stestr run {posargs}
+  stestr run --parallel-class {posargs}
   coverage combine
   coverage report --omit='*tests*'
   coverage html -d ./cover --omit='*tests*'