force python2.7 for tox pep8
force python2.7 for tox pep8

tox -re pep8 isn't working for systems that use python 3. Force tox
to use python2.7 for pep8.

Change-Id: I61468c3e3ab673dc84248dc72d1085c83ee57bc0
Closes-Bug: #1708163

stackforge/nova-powervm,openstack/nova-powervm,openstack/nova-powervm,stackforge/nova-powervm