Remove extra pep8/flake8/pyflakes requirements

pep8 flake8 and pyflakes are pinned as part of hacking.  So they don't
need to be pinned here as well.

Also add posargs to flake8, so can do things like 'tox -epep8 --
--statistics'

Change-Id: Id37949caa5a487cd98b6b85c026689fbd63cb5d8

test-requirements.txt

@@ -1,7 +1,3 @@
-# Install bounded pep8/pyflakes first, then let flake8 install
-pep8==1.4.5
-pyflakes==0.7.2
-flake8==2.0
 hacking>=0.5.6,<0.7
 coverage>=3.6
 discover

tox.ini

@@ -17,7 +17,7 @@ downloadcache = ~/cache/pip
 
 [testenv:pep8]
 commands =
-  flake8
+  flake8 {posargs}
 
 [testenv:cover]
 setenv = VIRTUAL_ENV={envdir}