Add back a bandit tox job

The bandit project uses an integration test to ensure it doesn't
break other projects by introducing new changes. To run this
integration, it is necessary to have a common tox target of
'bandit'.

Change-Id: I9b80043bde45cfec472402ddab1747d5b2f87da1

tox.ini

@@ -19,6 +19,11 @@ commands =
   flake8
   bandit -c bandit.yaml -r keystonemiddleware -n5 -p gate
 
+[testenv:bandit]
+# NOTE(browne): This is required for the integration test job of the bandit
+# project. Please do not remove.
+commands = bandit -c bandit.yaml -r keystonemiddleware -n5 -p gate
+
 [testenv:venv]
 commands = {posargs}