Debug env for tox

Running a test with pdb was difficult because tox captures
output and causes pdb prompt to quit.

Tips for how to run with debug are provided here:
 https://wiki.openstack.org/wiki/Testr#Debugging_.28pdb.29_Tests

This change puts these commands into a debug env in tox.ini so
you can do a command like:

tox -e debug pycadf.tests.test_cadf_spec

and when it hits your breakpoint you'll get the debug prompt.

Change-Id: Ic53a888e4e305d51159d1d6a2f962786a532e089
Co-Authored-By: Brant Knudson <bknudson@us.ibm.com>
Closes-Bug: #1350424
Merge "Debug env for tox"