Add debug to tox environment

The oslotest package distributes a shell file that may be used to assist
in debugging python code. The shell file uses testtools, and supports
debugging with pdb. Debug tox environment implements following test
instructions.
https://wiki.openstack.org/wiki/Testr#Debugging_.28pdb.29_Tests

To enable debugging, run tox with the debug environment. Below are the
following ways to run it.

 * tox -e debug module
 * tox -e debug module.test_class
 * tox -e debug module.test_class.test_method

Change-Id: I08937845803be7bd125b838ab07bda56f202e88d
Closes-Bug: 1632486

tox.ini

@@ -69,6 +69,11 @@ commands =
   find . -type f -name "*.pyc" -delete
   python setup.py testr --coverage --testr-args='{posargs}'
 
+[testenv:debug]
+commands =
+  find . -type f -name "*.pyc" -delete
+  oslo_debug_helper {posargs}
+
 [testenv:venv]
 commands = {posargs}