add local tox targets for pep8 and py3
add local tox targets for pep8 and py3

This patch adds local tox targets for pep8 and python 3 as per [1]
that will install dependencies in editable mode.
To run them use the pep8-dev and py3-dev targets respectively.

[1] https://etherpad.openstack.org/p/neutron-sibling-setup

Change-Id: I22e4ddc1d9a2e82bfb13302caf5be948607bb722

stackforge/tap-as-a-service,stackforge/tap-as-a-service,openstack/tap-as-a-service,openstack/tap-as-a-service