Make pep8 *the* linting interface

According to the PTI (=Python Test Interface,
http://governance.openstack.org/reference/cti/python_cti.html), pep8
is the interface for codestyle checks. Move all tests from linters to
pep8 and remove linters environment.

Change-Id: I9ed8a0504c05681f96a9ad8b81c4dbaeba56c62f
Depends-On: I9346e4bf8c3a50de088c6a157f0d871c0a9333a1

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 1.6
 skipsdist = True
-envlist = py34,py27,linters,releasenotes
+envlist = py34,py27,pep8,releasenotes
 
 [testenv]
 usedevelop = True
@@ -14,18 +14,11 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --testr-args='{posargs}'
 
-[testenv:linters]
+[testenv:pep8]
 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}