Add oslo_debug_helper to tox.ini
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: Iae0bc883e41dfa63a8b53bcfe037cc278a3c40ef

openstack/oslo.service,poznyakandrey/oslo.service