Add a debug env for unit tests
Add a debug env for unit tests

Debugging unit tests locally with pdb ends in failure with tox.
this patch adds debug environment to debug tests with pdb
break points by following below steps.

1. set break point in code by adding import pdb;pdb.set_trace
2. run tox -e debug test_regex

Change-Id: I088a558a1fb68b3bc2a218ccda0911ed1389f91f

mahak/neutron,noironetworks/neutron,openstack/neutron,openstack/neutron,mahak/neutron,noironetworks/neutron,openstack/neutron,mahak/neutron,huntxu/neutron,huntxu/neutron