Enable bandit

This commit enables bandit check and update the version to larger than
1.6.0 to prevent some issue.

Change-Id: I8da25bbb00ea2a6793b62924eb59c468b7384668

test-requirements.txt

@@ -4,5 +4,5 @@
 hacking>=3.0.1,<3.1.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 # Bandit security code scanner
-bandit>=1.1.0,<1.6.0 # Apache-2.0
+bandit>=1.6.0,<1.7.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0

tox.ini

@@ -18,8 +18,7 @@ commands =
 deps = {[testenv]deps}
 commands =
   flake8
-  # Run security linter (deactivated as it fails on tmpdir issue right now)
-  # bandit -r oslo_metrics -x tests -n5
+  bandit -r oslo_metrics -x tests -n5
 
 [testenv:venv]
 commands = {posargs}