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: I0439fa7e3923a4d3f8984024f2d4959c38563686

tox.ini

@@ -29,6 +29,9 @@ commands = {posargs}
 [testenv:cover]
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
+[testenv:debug]
+commands = oslo_debug_helper -t senlinclient/tests {posargs}
+
 [testenv:docs]
 commands=
     python setup.py build_sphinx