Add debug testenv in tox
Add debug testenv in tox

Once we add debug testenv, we can use "tox -e debug -- --debug"
to debug test cases when tox is running.

Change-Id: If12d6c7bd8b1ae04096b526ad37978f60fdb70db

vponomaryov/manila,scality/manila,bswartz/manila,bswartz/manila,NetApp/manila,openstack/manila,NetApp/manila,scality/manila,vponomaryov/manila,openstack/manila