Add debug testenv in tox
Add debug testenv in tox

Once we add debug testenv, we can use "tox -e debug" to debug test cases
when tox is running. Then tox will use oslotest which really debug our
test cases. Only we should do is insert pdb into code.
It's easy to use.

the same in magnum:
https://review.openstack.org/#/c/266020/

Closes-Bug: #1534030
Change-Id: I63ea8086e269b4ecacb7f7e337f1adf2a869db94

openstack/python-magnumclient