Add oslo_debug_helper to tox.ini

oslo_debug_helper is used to assist in debugging of python code,
i.e. by adding breakpoints via pdb.

To enable debugging, run tox with the debug environment:
tox -e debug <test_path>

Reference link:
http://docs.openstack.org/developer/oslotest/features.html#debugging-with-oslo-debug-helper

Change-Id: I01d66328a3529e542e503be2cd7bbca69a77c4aa

tox.ini

@@ -12,6 +12,9 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
+[testenv:debug]
+commands = oslo_debug_helper {posargs}
+
 [testenv:pep8]
 commands = flake8