Add version specific debug tox environments
Add version specific debug tox environments

Sometimes is useful to debug in a specific python version, this patch
adds two environments to allow such debugging:

  - debug-py27
  - debug-py35

Change-Id: I0f8439e6aa245628f0b27be5303415dc71f5c791

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl