Update tox debug env

Having no path in debug environment assumes the test path to be
neutron_tempest_plugin/tests which doesn't exists, so tox -e debug
debug fails complaining that neutron_tempest_plugin/tests isn't
importable. This patch fixes the test path for debug environment.

Change-Id: Iac685d4d4fcee357c40141acc79b2aac28bd535b
diff --git a/tox.ini b/tox.ini
index dbd0526..c16664d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -33,7 +33,7 @@
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
-commands = oslo_debug_helper {posargs}
+commands = oslo_debug_helper -t neutron_tempest_plugin/ {posargs}
 
 [flake8]
 # E125 continuation line does not distinguish itself from next logical line