Enable flake8 checks with tox

1. Report all style checks with source.
2. Also enable additional helper checks providined in openstack-dev/hacking.

To run pep checks:
tox -e pep8

Change-Id: I46f020bafcea97305d4dc03a3c34040e0d70092a

CONTRIBUTING.rst

@@ -0,0 +1,16 @@
+If you would like to contribute to the development of OpenStack,
+you must follow the steps in this page:
+
+   http://docs.openstack.org/infra/manual/developers.html
+
+Once those steps have been completed, changes to OpenStack
+should be submitted for review via the Gerrit tool, following
+the workflow documented at:
+
+   http://docs.openstack.org/infra/manual/developers.html#development-workflow
+
+Pull requests submitted through GitHub will be ignored.
+
+Bugs should be filed on Launchpad, not in GitHub's issue tracker:
+
+   https://bugs.launchpad.net/omni

test-requirements.txt

@@ -0,0 +1 @@
+hacking>=0.12.0,<0.13 # Apache-2.0

tox.ini

@@ -1,11 +1,23 @@
 [tox]
-envlist = py27
+envlist = py27,pep8
-skipsdist = true
+skipsdist = True
 minversion = 2.3.2
 
 [testenv]
 usedevelop = True
+deps = -r{toxinidir}/test-requirements.txt
 whitelist_externals = bash
 commands =
     bash clone_repos.sh
     bash run_tests.sh -wj
+
+[testenv:pep8]
+commands = flake8 {posargs}
+
+[flake8]
+# E123, E125 skipped as they are invalid PEP-8.
+show-source = True
+enable-extensions = H106,H203
+ignore = E123,E125
+builtins = _
+exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build