Add output of slowest tests to UT runs

With the switch to stestr we lost the automatic output of the slowest
tests. We can still get that output, but it is a separate command to
run after the tests. Add that to our tox.ini command for test execution.

Change-Id: I37937c1ebe4a10b896f1deb77f64b520ceba2830

tox.ini

@@ -25,6 +25,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands =
   find . -type f -name "*.pyc" -delete
   stestr run '{posargs}'
+  stestr slowest
 
 whitelist_externals =
   bash