always use python2.7 for pep8
always use python2.7 for pep8

pep8 doesn't work with python3 on our codebase. If someone is on a
platform that defaults to python => python3, pep8 won't work for
them. This is actually really easy to fix with a single line in tox.

Change-Id: I7a888e4f7cc828638a9d61d2249a854ba1f3cb7b
Closes-Bug: #1456955

dims/nova,CEG-FYP-OpenStack/scheduler,klmitch/nova,gooddata/openstack-nova,klmitch/nova,mikalstill/nova,cyx1231st/nova,dims/nova,cernops/nova,gooddata/openstack-nova,jianghuaw/nova,BeyondTheClouds/nova,Juniper/nova,Juniper/nova,klmitch/nova,cloudbase/nova,rahulunair/nova,zhimin711/nova,mahak/nova,openstack/nova,openstack/nova,vmturbo/nova,BeyondTheClouds/nova,BeyondTheClouds/nova,klmitch/nova,mahak/nova,Juniper/nova,vmturbo/nova,NeCTAR-RC/nova,vmturbo/nova,mikalstill/nova,cloudbase/nova,sebrandon1/nova,sebrandon1/nova,rajalokan/nova,rahulunair/nova,alaski/nova,alaski/nova,cernops/nova,phenoxim/nova,hanlind/nova,gooddata/openstack-nova,jianghuaw/nova,openstack/nova,cernops/nova,bigswitch/nova,phenoxim/nova,rahulunair/nova,CEG-FYP-OpenStack/scheduler,bigswitch/nova,zhimin711/nova,vmturbo/nova,jianghuaw/nova,cyx1231st/nova,Juniper/nova,hanlind/nova,rajalokan/nova,cloudbase/nova,NeCTAR-RC/nova,mahak/nova,jianghuaw/nova,mikalstill/nova,hanlind/nova,gooddata/openstack-nova,sebrandon1/nova,rajalokan/nova,rajalokan/nova