Switch functional jobs to python 3
Switch functional jobs to python 3

As tracked in [0], functional job is the last one not running on
python 3 yet. With new zuul v3 job definition in parent, switch default
and create a tox target for python 2.7

[0] https://etherpad.openstack.org/p/neutron_stadium_python3_status

Change-Id: Ifb3b4e1bb7b20c77727d3371bc058ff2cff0c945

openstack/networking-sfc,openstack/networking-sfc