Make bandit voting as part of pep8

Run security linter bandit as part of pep8. Pep8 is the usual linter
target and thus let's use it there instead of starting another node for
this short-running job.

The job is running stable and thus this can be made voting.

A followup change will remove the non-voting bandit job.

Change-Id: I6f2cf3902a4e786a5d31a06bf66d19bc33a60b55

tox.ini

@@ -63,7 +63,10 @@ commands =
 
 [testenv:pep8]
 sitepackages = False
-commands = flake8 {posargs}
+commands =
+  flake8 {posargs}
+  # Run security linter
+  bandit -c bandit.yaml -r barbican -n5 -p barbican_conservative
 
 [testenv:venv]
 commands = {posargs}