Add pdb support to tox with debug helper shell script

The Keystone team has been using a home brewed `debug_helper.sh`
file to run tests with pdb support, it's now being also used by
pycadf too. As noted by bnemec [1] we should move this to oslo.
[1] https://bugs.launchpad.net/pycadf/+bug/1354192

To run any test in particular, run tox with -e debug:
  `tox -e debug`

It also supports passing in a specific test module, class or case.
The shell file runs testtools underneath the covers to get pdb
support.

Change-Id: Idb715bc137459a2f6d16ac3f65c718a567df49ff
Co-Authored-By: Brant Knudson <bknudson@us.ibm.com>