Add debug to tox environment

At present oslo_debug_helper assumes that the importable directory
is os-brick and fails unless overriden with :

  $ tox -e debug -- -t os_brick

Change-Id: Ic8dd164d8515dd58b4456699e5ed114815ff8c4f

tox.ini

@@ -21,8 +21,14 @@ deps = -r{toxinidir}/requirements.txt
 commands = ostestr {posargs}
 
 whitelist_externals = bash
+                      find
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
+[testenv:debug]
+commands =
+  find . -type f -name "*.pyc" -delete
+  oslo_debug_helper {posargs}
+
 [testenv:pep8]
 commands = flake8 {posargs}