Use pep8 instead of linters

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.

Change-Id: I9b53b091f69290e3bda43ee1d50122c39b054e1b

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = linters
+envlist = pep8
 
 [testenv]
 usedevelop = True
@@ -9,7 +9,7 @@ deps = -r{toxinidir}/test-requirements.txt
 setenv = VIRTUAL_ENV={envdir}
 sitepackages = False
 
-[testenv:linters]
+[testenv:pep8]
 commands =
     flake8 openstackdocstheme