Use oslo_debug_helper and remove our own version

With the latest version of oslotest, we can now take advantage
of a common oslo_debug_helper script.
We can now remove our own homebrewed version, minor changes to
tox.ini were needed.

Change-Id: Ifd5d0454e424ecfca0db30d3d3f6350ba7db1b60
Merge "Use oslo_debug_helper and remove our own version"