Add slowest command to tox.ini

This commit adds a printing of the 10 slowest tests during the run at
the end of the tox jobs. People have found this useful in the past and
ostestr used to do this by defualt. But, now that stestr is used to run
tests we need to make this call after a run to retain the output.

Change-Id: Ife81963e5ffb63e4ea09817e735eb768d7cf6209

tox.ini

@@ -31,6 +31,7 @@ commands =
     {[testenv]commands}
     stestr run '{posargs}'
     env TEST_OSPROFILER=1 stestr run --combine --no-discover 'nova.tests.unit.test_profiler'
+    stestr slowest
 
 [testenv:py35]
 commands =
@@ -69,6 +70,7 @@ setenv = VIRTUAL_ENV={envdir}
 commands =
   find . -type f -name "*.pyc" -delete
   stestr run '{posargs}'
+  stestr slowest
 
 # TODO(gcb) Merge this into [testenv:functional] when functional tests are gating
 # with python 3.5
@@ -92,6 +94,7 @@ setenv = VIRTUAL_ENV={envdir}
 commands =
   find . -type f -name "*.pyc" -delete
   stestr run '{posargs}'
+  stestr slowest
 
 [testenv:genconfig]
 commands = oslo-config-generator --config-file=etc/nova/nova-config-generator.conf