Merge pep8 and bandit into linters

Create new linters environment consisting of pep8 and bandit.
Keep pep8 as an alias for linters and add a message when invoked.

Once this change is in, the infra job can be changed to only run
linters job, this allows to run one less job for each change.

Change-Id: Idff7d7134c50aa4740ed87eeeda83f7cca32f3db

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 1.6
 skipsdist = True
-envlist = py34,py27,pep8,releasenotes
+envlist = py34,py27,linters,releasenotes
 
 [testenv]
 usedevelop = True
@@ -14,9 +14,17 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --testr-args='{posargs}'
 
-[testenv:pep8]
+[testenv:linters]
 commands =
   flake8
+  bandit -c bandit.yaml -r keystonemiddleware -n5 -p gate
+
+[testenv:pep8]
+whitelist_externals =
+  echo
+commands =
+  {[testenv:linters]commands}
+  echo "Use tox -e linters instead"
 
 [testenv:venv]
 commands = {posargs}