Switch neutron-rally-task job to python 3
Switch neutron-rally-task job to python 3

Depends-On: https://review.openstack.org/624409/

Change-Id: I5d6f3616fe5fa5a9cbd57a9dc3124a9ec4646a7f

openstack/neutron,noironetworks/neutron,mahak/neutron,mahak/neutron,noironetworks/neutron,openstack/neutron,mahak/neutron,openstack/neutron