Add debug environment to tox

The oslo_debug_helper enables developers to use pdb for debugging the
unittests.

Ref: http://docs.openstack.org/developer/oslotest/features.html

Change-Id: I08d4c0fcb2e8015c02f5e9b9d4e9dff7f58d21ea

doc/source/dev/dev-quickstart.rst

@@ -116,6 +116,26 @@ To run only the pep8/flake8 syntax and style checks::
 
     tox -epep8
 
+Debugging unit tests
+--------------------
+
+In order to break into the debugger from a unit test we need to insert
+a breaking point to the code:
+
+.. code-block:: python
+
+  import pdb; pdb.set_trace()
+
+Then run ``tox`` with the debug environment as one of the following::
+
+  tox -e debug
+  tox -e debug test_file_name
+  tox -e debug test_file_name.TestClass
+  tox -e debug test_file_name.TestClass.test_name
+
+For more information see the `oslotest documentation
+<http://docs.openstack.org/developer/oslotest/features.html#debugging-with-oslo-debug-helper>`_.
+
 ===============================
 Exercising the Services Locally
 ===============================

tox.ini

@@ -58,6 +58,9 @@ envdir = {toxworkdir}/venv
 commands =
   oslo-config-generator --config-file=tools/config/ironic-config-generator.conf
 
+[testenv:debug]
+commands = oslo_debug_helper -t ironic/tests/unit {posargs}
+
 [testenv:docs]
 setenv = PYTHONHASHSEED=0
 sitepackages = False